src/gromacs/gmxlib/ewald_util.c

   1 /*
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  37 #ifdef HAVE_CONFIG_H
  38 #include <config.h>
  39 #endif
  40
  41 #include <stdio.h>
  42 #include <math.h>
  43 #include "gromacs/math/utilities.h"
  44 #include "typedefs.h"
  45 #include "types/commrec.h"
  46 #include "vec.h"
  47 #include "coulomb.h"
  48 #include "gromacs/utility/smalloc.h"
  49 #include "physics.h"
  50 #include "txtdump.h"
  51 #include "gromacs/fileio/futil.h"
  52 #include "names.h"
  53 #include "macros.h"
  54
  55 real calc_ewaldcoeff_q(real rc, real dtol)
  56 {
  57     real x = 5, low, high;
  58     int  n, i = 0;
  59
  60
  61     do
  62     {
  63         i++;
  64         x *= 2;
  65     }
  66     while (gmx_erfc(x*rc) > dtol);
  67
  68     n    = i+60; /* search tolerance is 2^-60 */
  69     low  = 0;
  70     high = x;
  71     for (i = 0; i < n; i++)
  72     {
  73         x = (low+high)/2;
  74         if (gmx_erfc(x*rc) > dtol)
  75         {
  76             low = x;
  77         }
  78         else
  79         {
  80             high = x;
  81         }
  82     }
  83     return x;
  84 }
  85
  86 static real ewald_function_lj(real x, real rc)
  87 {
  88     real xrc, xrc2, xrc4, factor;
  89     xrc  = x*rc;
  90     xrc2 = xrc*xrc;
  91     xrc4 = xrc2*xrc2;
  92 #ifdef GMX_DOUBLE
  93     factor = exp(-xrc2)*(1 + xrc2 + xrc4/2.0);
  94 #else
  95     factor = expf(-xrc2)*(1 + xrc2 + xrc4/2.0);
  96 #endif
  97
  98     return factor;
  99 }
 100
 101 real calc_ewaldcoeff_lj(real rc, real dtol)
 102 {
 103     real x = 5, low, high;
 104     int  n, i = 0;
 105
 106     do
 107     {
 108         i++;
 109         x *= 2.0;
 110     }
 111     while (ewald_function_lj(x, rc) > dtol);
 112
 113     n    = i + 60; /* search tolerance is 2^-60 */
 114     low  = 0;
 115     high = x;
 116     for (i = 0; i < n; ++i)
 117     {
 118         x = (low + high) / 2.0;
 119         if (ewald_function_lj(x, rc) > dtol)
 120         {
 121             low = x;
 122         }
 123         else
 124         {
 125             high = x;
 126         }
 127     }
 128     return x;
 129 }
 130
 131 /* There's nothing special to do here if just masses are perturbed,
 132  * but if either charge or type is perturbed then the implementation
 133  * requires that B states are defined for both charge and type, and
 134  * does not optimize for the cases where only one changes.
 135  *
 136  * The parameter vectors for B states are left undefined in atoms2md()
 137  * when either FEP is inactive, or when there are no mass/charge/type
 138  * perturbations. The parameter vectors for LJ-PME are likewise
 139  * undefined when LJ-PME is not active. This works because
 140  * bHaveChargeOrTypePerturbed handles the control flow. */
 141 void ewald_LRcorrection(int start, int end,
 142                         t_commrec *cr, int thread, t_forcerec *fr,
 143                         real *chargeA, real *chargeB,
 144                         real *C6A, real *C6B,
 145                         real *sigmaA, real *sigmaB,
 146                         real *sigma3A, real *sigma3B,
 147                         gmx_bool bHaveChargeOrTypePerturbed,
 148                         gmx_bool calc_excl_corr,
 149                         t_blocka *excl, rvec x[],
 150                         matrix box, rvec mu_tot[],
 151                         int ewald_geometry, real epsilon_surface,
 152                         rvec *f, tensor vir_q, tensor vir_lj,
 153                         real *Vcorr_q, real *Vcorr_lj,
 154                         real lambda_q, real lambda_lj,
 155                         real *dvdlambda_q, real *dvdlambda_lj)
 156 {
 157     int         i, i1, i2, j, k, m, iv, jv, q;
 158     atom_id    *AA;
 159     double      Vexcl_q, dvdl_excl_q, dvdl_excl_lj; /* Necessary for precision */
 160     double      Vexcl_lj;
 161     real        one_4pi_eps;
 162     real        v, vc, qiA, qiB, dr2, rinv, enercorr;
 163     real        Vself_q[2], Vself_lj[2], Vdipole[2], rinv2, ewc_q = fr->ewaldcoeff_q, ewcdr;
 164     real        ewc_lj = fr->ewaldcoeff_lj, ewc_lj2 = ewc_lj * ewc_lj;
 165     real        c6Ai   = 0, c6Bi = 0, c6A = 0, c6B = 0, ewcdr2, ewcdr4, c6L = 0, rinv6;
 166     rvec        df, dx, mutot[2], dipcorrA, dipcorrB;
 167     tensor      dxdf_q, dxdf_lj;
 168     real        vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 169     real        L1_q, L1_lj, dipole_coeff, qqA, qqB, qqL, vr0_q, vr0_lj = 0;
 170     gmx_bool    bMolPBC      = fr->bMolPBC;
 171     gmx_bool    bDoingLBRule = (fr->ljpme_combination_rule == eljpmeLB);
 172
 173     /* This routine can be made faster by using tables instead of analytical interactions
 174      * However, that requires a thorough verification that they are correct in all cases.
 175      */
 176
 177     one_4pi_eps   = ONE_4PI_EPS0/fr->epsilon_r;
 178     vr0_q         = ewc_q*M_2_SQRTPI;
 179     if (EVDW_PME(fr->vdwtype))
 180     {
 181         vr0_lj    = -pow(ewc_lj, 6)/6.0;
 182     }
 183
 184     AA           = excl->a;
 185     Vexcl_q      = 0;
 186     Vexcl_lj     = 0;
 187     dvdl_excl_q  = 0;
 188     dvdl_excl_lj = 0;
 189     Vdipole[0]   = 0;
 190     Vdipole[1]   = 0;
 191     L1_q         = 1.0-lambda_q;
 192     L1_lj        = 1.0-lambda_lj;
 193     /* Note that we have to transform back to gromacs units, since
 194      * mu_tot contains the dipole in debye units (for output).
 195      */
 196     for (i = 0; (i < DIM); i++)
 197     {
 198         mutot[0][i] = mu_tot[0][i]*DEBYE2ENM;
 199         mutot[1][i] = mu_tot[1][i]*DEBYE2ENM;
 200         dipcorrA[i] = 0;
 201         dipcorrB[i] = 0;
 202     }
 203     dipole_coeff = 0;
 204     switch (ewald_geometry)
 205     {
 206         case eewg3D:
 207             if (epsilon_surface != 0)
 208             {
 209                 dipole_coeff =
 210                     2*M_PI*ONE_4PI_EPS0/((2*epsilon_surface + fr->epsilon_r)*vol);
 211                 for (i = 0; (i < DIM); i++)
 212                 {
 213                     dipcorrA[i] = 2*dipole_coeff*mutot[0][i];
 214                     dipcorrB[i] = 2*dipole_coeff*mutot[1][i];
 215                 }
 216             }
 217             break;
 218         case eewg3DC:
 219             dipole_coeff = 2*M_PI*one_4pi_eps/vol;
 220             dipcorrA[ZZ] = 2*dipole_coeff*mutot[0][ZZ];
 221             dipcorrB[ZZ] = 2*dipole_coeff*mutot[1][ZZ];
 222             break;
 223         default:
 224             gmx_incons("Unsupported Ewald geometry");
 225             break;
 226     }
 227     if (debug)
 228     {
 229         fprintf(debug, "dipcorr = %8.3f  %8.3f  %8.3f\n",
 230                 dipcorrA[XX], dipcorrA[YY], dipcorrA[ZZ]);
 231         fprintf(debug, "mutot   = %8.3f  %8.3f  %8.3f\n",
 232                 mutot[0][XX], mutot[0][YY], mutot[0][ZZ]);
 233     }
 234     clear_mat(dxdf_q);
 235     if (EVDW_PME(fr->vdwtype))
 236     {
 237         clear_mat(dxdf_lj);
 238     }
 239     if ((calc_excl_corr || dipole_coeff != 0) && !bHaveChargeOrTypePerturbed)
 240     {
 241         for (i = start; (i < end); i++)
 242         {
 243             /* Initiate local variables (for this i-particle) to 0 */
 244             qiA = chargeA[i]*one_4pi_eps;
 245             if (EVDW_PME(fr->vdwtype))
 246             {
 247                 c6Ai = C6A[i];
 248                 if (bDoingLBRule)
 249                 {
 250                     c6Ai *= sigma3A[i];
 251                 }
 252             }
 253             if (calc_excl_corr)
 254             {
 255                 i1  = excl->index[i];
 256                 i2  = excl->index[i+1];
 257
 258                 /* Loop over excluded neighbours */
 259                 for (j = i1; (j < i2); j++)
 260                 {
 261                     k = AA[j];
 262                     /*
 263                      * First we must test whether k <> i, and then,
 264                      * because the exclusions are all listed twice i->k
 265                      * and k->i we must select just one of the two.  As
 266                      * a minor optimization we only compute forces when
 267                      * the charges are non-zero.
 268                      */
 269                     if (k > i)
 270                     {
 271                         qqA = qiA*chargeA[k];
 272                         if (EVDW_PME(fr->vdwtype))
 273                         {
 274                             c6A  = c6Ai * C6A[k];
 275                             if (bDoingLBRule)
 276                             {
 277                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 278                             }
 279                         }
 280                         if (qqA != 0.0 || c6A != 0.0)
 281                         {
 282                             real fscal;
 283
 284                             fscal = 0;
 285                             rvec_sub(x[i], x[k], dx);
 286                             if (bMolPBC)
 287                             {
 288                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 289                                 for (m = DIM-1; (m >= 0); m--)
 290                                 {
 291                                     if (dx[m] > 0.5*box[m][m])
 292                                     {
 293                                         rvec_dec(dx, box[m]);
 294                                     }
 295                                     else if (dx[m] < -0.5*box[m][m])
 296                                     {
 297                                         rvec_inc(dx, box[m]);
 298                                     }
 299                                 }
 300                             }
 301                             dr2 = norm2(dx);
 302                             /* Distance between two excluded particles
 303                              * may be zero in the case of shells
 304                              */
 305                             if (dr2 != 0)
 306                             {
 307                                 rinv              = gmx_invsqrt(dr2);
 308                                 rinv2             = rinv*rinv;
 309                                 if (qqA != 0.0)
 310                                 {
 311                                     real dr;
 312
 313                                     dr       = 1.0/rinv;
 314                                     ewcdr    = ewc_q*dr;
 315                                     vc       = qqA*gmx_erf(ewcdr)*rinv;
 316                                     Vexcl_q += vc;
 317 #ifdef GMX_DOUBLE
 318                                     /* Relative accuracy at R_ERF_R_INACC of 3e-10 */
 319 #define       R_ERF_R_INACC 0.006
 320 #else
 321                                     /* Relative accuracy at R_ERF_R_INACC of 2e-5 */
 322 #define       R_ERF_R_INACC 0.1
 323 #endif
 324                                     /* fscal is the scalar force pre-multiplied by rinv,
 325                                      * to normalise the relative position vector dx */
 326                                     if (ewcdr > R_ERF_R_INACC)
 327                                     {
 328                                         fscal = rinv2*(vc - qqA*ewc_q*M_2_SQRTPI*exp(-ewcdr*ewcdr));
 329                                     }
 330                                     else
 331                                     {
 332                                         /* Use a fourth order series expansion for small ewcdr */
 333                                         fscal = ewc_q*ewc_q*qqA*vr0_q*(2.0/3.0 - 0.4*ewcdr*ewcdr);
 334                                     }
 335
 336                                     /* The force vector is obtained by multiplication with
 337                                      * the relative position vector
 338                                      */
 339                                     svmul(fscal, dx, df);
 340                                     rvec_inc(f[k], df);
 341                                     rvec_dec(f[i], df);
 342                                     for (iv = 0; (iv < DIM); iv++)
 343                                     {
 344                                         for (jv = 0; (jv < DIM); jv++)
 345                                         {
 346                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 347                                         }
 348                                     }
 349                                 }
 350
 351                                 if (c6A != 0.0)
 352                                 {
 353                                     rinv6     = rinv2*rinv2*rinv2;
 354                                     ewcdr2    = ewc_lj2*dr2;
 355                                     ewcdr4    = ewcdr2*ewcdr2;
 356                                     /* We get the excluded long-range contribution from -C6*(1-g(r))
 357                                      * g(r) is also defined in the manual under LJ-PME
 358                                      */
 359                                     vc        = -c6A*rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 360                                     Vexcl_lj += vc;
 361                                     /* The force is the derivative of the potential vc.
 362                                      * fscal is the scalar force pre-multiplied by rinv,
 363                                      * to normalise the relative position vector dx */
 364                                     fscal     = 6.0*vc*rinv2 + c6A*rinv6*exp(-ewcdr2)*ewc_lj2*ewcdr4;
 365
 366                                     /* The force vector is obtained by multiplication with
 367                                      * the relative position vector
 368                                      */
 369                                     svmul(fscal, dx, df);
 370                                     rvec_inc(f[k], df);
 371                                     rvec_dec(f[i], df);
 372                                     for (iv = 0; (iv < DIM); iv++)
 373                                     {
 374                                         for (jv = 0; (jv < DIM); jv++)
 375                                         {
 376                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 377                                         }
 378                                     }
 379                                 }
 380                             }
 381                             else
 382                             {
 383                                 Vexcl_q  += qqA*vr0_q;
 384                                 Vexcl_lj += c6A*vr0_lj;
 385                             }
 386                         }
 387                     }
 388                 }
 389             }
 390             /* Dipole correction on force */
 391             if (dipole_coeff != 0)
 392             {
 393                 for (j = 0; (j < DIM); j++)
 394                 {
 395                     f[i][j] -= dipcorrA[j]*chargeA[i];
 396                 }
 397             }
 398         }
 399     }
 400     else if (calc_excl_corr || dipole_coeff != 0)
 401     {
 402         for (i = start; (i < end); i++)
 403         {
 404             /* Initiate local variables (for this i-particle) to 0 */
 405             qiA = chargeA[i]*one_4pi_eps;
 406             qiB = chargeB[i]*one_4pi_eps;
 407             if (EVDW_PME(fr->vdwtype))
 408             {
 409                 c6Ai = C6A[i];
 410                 c6Bi = C6B[i];
 411                 if (bDoingLBRule)
 412                 {
 413                     c6Ai *= sigma3A[i];
 414                     c6Bi *= sigma3B[i];
 415                 }
 416             }
 417             if (calc_excl_corr)
 418             {
 419                 i1  = excl->index[i];
 420                 i2  = excl->index[i+1];
 421
 422                 /* Loop over excluded neighbours */
 423                 for (j = i1; (j < i2); j++)
 424                 {
 425                     k = AA[j];
 426                     if (k > i)
 427                     {
 428                         qqA = qiA*chargeA[k];
 429                         qqB = qiB*chargeB[k];
 430                         if (EVDW_PME(fr->vdwtype))
 431                         {
 432                             c6A = c6Ai*C6A[k];
 433                             c6B = c6Bi*C6B[k];
 434                             if (bDoingLBRule)
 435                             {
 436                                 c6A *= pow(0.5*(sigmaA[i]+sigmaA[k]), 6)*sigma3A[k];
 437                                 c6B *= pow(0.5*(sigmaB[i]+sigmaB[k]), 6)*sigma3B[k];
 438                             }
 439                         }
 440                         if (qqA != 0.0 || qqB != 0.0 || c6A != 0.0 || c6B != 0.0)
 441                         {
 442                             real fscal;
 443
 444                             fscal = 0;
 445                             qqL   = L1_q*qqA + lambda_q*qqB;
 446                             if (EVDW_PME(fr->vdwtype))
 447                             {
 448                                 c6L = L1_lj*c6A + lambda_lj*c6B;
 449                             }
 450                             rvec_sub(x[i], x[k], dx);
 451                             if (bMolPBC)
 452                             {
 453                                 /* Cheap pbc_dx, assume excluded pairs are at short distance. */
 454                                 for (m = DIM-1; (m >= 0); m--)
 455                                 {
 456                                     if (dx[m] > 0.5*box[m][m])
 457                                     {
 458                                         rvec_dec(dx, box[m]);
 459                                     }
 460                                     else if (dx[m] < -0.5*box[m][m])
 461                                     {
 462                                         rvec_inc(dx, box[m]);
 463                                     }
 464                                 }
 465                             }
 466                             dr2 = norm2(dx);
 467                             if (dr2 != 0)
 468                             {
 469                                 rinv    = gmx_invsqrt(dr2);
 470                                 rinv2   = rinv*rinv;
 471                                 if (qqA != 0.0 || qqB != 0.0)
 472                                 {
 473                                     real dr;
 474
 475                                     dr           = 1.0/rinv;
 476                                     v            = gmx_erf(ewc_q*dr)*rinv;
 477                                     vc           = qqL*v;
 478                                     Vexcl_q     += vc;
 479                                     /* fscal is the scalar force pre-multiplied by rinv,
 480                                      * to normalise the relative position vector dx */
 481                                     fscal        = rinv2*(vc-qqL*ewc_q*M_2_SQRTPI*exp(-ewc_q*ewc_q*dr2));
 482                                     dvdl_excl_q += (qqB - qqA)*v;
 483
 484                                     /* The force vector is obtained by multiplication with
 485                                      * the relative position vector
 486                                      */
 487                                     svmul(fscal, dx, df);
 488                                     rvec_inc(f[k], df);
 489                                     rvec_dec(f[i], df);
 490                                     for (iv = 0; (iv < DIM); iv++)
 491                                     {
 492                                         for (jv = 0; (jv < DIM); jv++)
 493                                         {
 494                                             dxdf_q[iv][jv] += dx[iv]*df[jv];
 495                                         }
 496                                     }
 497                                 }
 498
 499                                 if ((c6A != 0.0 || c6B != 0.0) && EVDW_PME(fr->vdwtype))
 500                                 {
 501                                     rinv6         = rinv2*rinv2*rinv2;
 502                                     ewcdr2        = ewc_lj2*dr2;
 503                                     ewcdr4        = ewcdr2*ewcdr2;
 504                                     v             = -rinv6*(1.0 - exp(-ewcdr2)*(1 + ewcdr2 + 0.5*ewcdr4));
 505                                     vc            = c6L*v;
 506                                     Vexcl_lj     += vc;
 507                                     /* fscal is the scalar force pre-multiplied by rinv,
 508                                      * to normalise the relative position vector dx */
 509                                     fscal         = 6.0*vc*rinv2 + c6L*rinv6*exp(-ewcdr2)*ewc_lj2*ewcdr4;
 510                                     dvdl_excl_lj += (c6B - c6A)*v;
 511
 512                                     /* The force vector is obtained by multiplication with
 513                                      * the relative position vector
 514                                      */
 515                                     svmul(fscal, dx, df);
 516                                     rvec_inc(f[k], df);
 517                                     rvec_dec(f[i], df);
 518                                     for (iv = 0; (iv < DIM); iv++)
 519                                     {
 520                                         for (jv = 0; (jv < DIM); jv++)
 521                                         {
 522                                             dxdf_lj[iv][jv] += dx[iv]*df[jv];
 523                                         }
 524                                     }
 525                                 }
 526                             }
 527                             else
 528                             {
 529                                 Vexcl_q      += qqL*vr0_q;
 530                                 dvdl_excl_q  += (qqB - qqA)*vr0_q;
 531                                 Vexcl_lj     += c6L*vr0_lj;
 532                                 dvdl_excl_lj += (c6B - c6A)*vr0_lj;
 533                             }
 534                         }
 535                     }
 536                 }
 537             }
 538             /* Dipole correction on force */
 539             if (dipole_coeff != 0)
 540             {
 541                 for (j = 0; (j < DIM); j++)
 542                 {
 543                     f[i][j] -= L1_q*dipcorrA[j]*chargeA[i]
 544                         + lambda_q*dipcorrB[j]*chargeB[i];
 545                 }
 546             }
 547         }
 548     }
 549     for (iv = 0; (iv < DIM); iv++)
 550     {
 551         for (jv = 0; (jv < DIM); jv++)
 552         {
 553             vir_q[iv][jv]  += 0.5*dxdf_q[iv][jv];
 554             vir_lj[iv][jv] += 0.5*dxdf_lj[iv][jv];
 555         }
 556     }
 557
 558     Vself_q[0]  = 0;
 559     Vself_q[1]  = 0;
 560     Vself_lj[0] = 0;
 561     Vself_lj[1] = 0;
 562
 563     /* Global corrections only on master process */
 564     if (MASTER(cr) && thread == 0)
 565     {
 566         for (q = 0; q < (bHaveChargeOrTypePerturbed ? 2 : 1); q++)
 567         {
 568             if (calc_excl_corr)
 569             {
 570                 /* Self-energy correction */
 571                 Vself_q[q] = ewc_q*one_4pi_eps*fr->q2sum[q]*M_1_SQRTPI;
 572                 if (EVDW_PME(fr->vdwtype))
 573                 {
 574                     Vself_lj[q] =  fr->c6sum[q]*0.5*vr0_lj;
 575                 }
 576             }
 577
 578             /* Apply surface dipole correction:
 579              * correction = dipole_coeff * (dipole)^2
 580              */
 581             if (dipole_coeff != 0)
 582             {
 583                 if (ewald_geometry == eewg3D)
 584                 {
 585                     Vdipole[q] = dipole_coeff*iprod(mutot[q], mutot[q]);
 586                 }
 587                 else if (ewald_geometry == eewg3DC)
 588                 {
 589                     Vdipole[q] = dipole_coeff*mutot[q][ZZ]*mutot[q][ZZ];
 590                 }
 591             }
 592         }
 593     }
 594     if (!bHaveChargeOrTypePerturbed)
 595     {
 596         *Vcorr_q = Vdipole[0] - Vself_q[0] - Vexcl_q;
 597         if (EVDW_PME(fr->vdwtype))
 598         {
 599             *Vcorr_lj = -Vself_lj[0] - Vexcl_lj;
 600         }
 601     }
 602     else
 603     {
 604         *Vcorr_q = L1_q*(Vdipole[0] - Vself_q[0])
 605             + lambda_q*(Vdipole[1] - Vself_q[1])
 606             - Vexcl_q;
 607         *dvdlambda_q += Vdipole[1] - Vself_q[1]
 608             - (Vdipole[0] - Vself_q[0]) - dvdl_excl_q;
 609         if (EVDW_PME(fr->vdwtype))
 610         {
 611             *Vcorr_lj      = -(L1_lj*Vself_lj[0] + lambda_lj*Vself_lj[1]) - Vexcl_lj;
 612             *dvdlambda_lj += -Vself_lj[1] + Vself_lj[0] - dvdl_excl_lj;
 613         }
 614     }
 615
 616     if (debug)
 617     {
 618         fprintf(debug, "Long Range corrections for Ewald interactions:\n");
 619         fprintf(debug, "start=%d,natoms=%d\n", start, end-start);
 620         fprintf(debug, "q2sum = %g, Vself_q=%g c6sum = %g, Vself_lj=%g\n",
 621                 L1_q*fr->q2sum[0]+lambda_q*fr->q2sum[1], L1_q*Vself_q[0]+lambda_q*Vself_q[1], L1_lj*fr->c6sum[0]+lambda_lj*fr->c6sum[1], L1_lj*Vself_lj[0]+lambda_lj*Vself_lj[1]);
 622         fprintf(debug, "Electrostatic Long Range correction: Vexcl=%g\n", Vexcl_q);
 623         fprintf(debug, "Lennard-Jones Long Range correction: Vexcl=%g\n", Vexcl_lj);
 624         if (MASTER(cr) && thread == 0)
 625         {
 626             if (epsilon_surface > 0 || ewald_geometry == eewg3DC)
 627             {
 628                 fprintf(debug, "Total dipole correction: Vdipole=%g\n",
 629                         L1_q*Vdipole[0]+lambda_q*Vdipole[1]);
 630             }
 631         }
 632     }
 633 }
 634
 635 real ewald_charge_correction(t_commrec *cr, t_forcerec *fr, real lambda,
 636                              matrix box,
 637                              real *dvdlambda, tensor vir)
 638
 639 {
 640     real vol, fac, qs2A, qs2B, vc, enercorr;
 641     int  d;
 642
 643     if (MASTER(cr))
 644     {
 645         /* Apply charge correction */
 646         vol = box[XX][XX]*box[YY][YY]*box[ZZ][ZZ];
 647
 648         fac = M_PI*ONE_4PI_EPS0/(fr->epsilon_r*2.0*vol*vol*sqr(fr->ewaldcoeff_q));
 649
 650         qs2A = fr->qsum[0]*fr->qsum[0];
 651         qs2B = fr->qsum[1]*fr->qsum[1];
 652
 653         vc = (qs2A*(1 - lambda) + qs2B*lambda)*fac;
 654
 655         enercorr = -vol*vc;
 656
 657         *dvdlambda += -vol*(qs2B - qs2A)*fac;
 658
 659         for (d = 0; d < DIM; d++)
 660         {
 661             vir[d][d] += vc;
 662         }
 663
 664         if (debug)
 665         {
 666             fprintf(debug, "Total charge correction: Vcharge=%g\n", enercorr);
 667         }
 668     }
 669     else
 670     {
 671         enercorr = 0;
 672     }
 673
 674     return enercorr;
 675 }