src/gromacs/ewald/ewald_utils.cpp

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  37 #include "gmxpre.h"
  38
  39 #include "ewald_utils.h"
  40
  41 #include <cmath>
  42
  43 #include "gromacs/math/utilities.h"
  44 #include "gromacs/utility/real.h"
  45
  46 real calc_ewaldcoeff_q(real rc, real rtol)
  47 {
  48     real beta = 5, low, high;
  49     int  n, i = 0;
  50
  51     do
  52     {
  53         i++;
  54         beta *= 2;
  55     } while (std::erfc(beta * rc) > rtol);
  56
  57     /* Do a binary search with tolerance 2^-60 */
  58     n    = i + 60;
  59     low  = 0;
  60     high = beta;
  61     for (i = 0; i < n; i++)
  62     {
  63         beta = (low + high) / 2;
  64         if (std::erfc(beta * rc) > rtol)
  65         {
  66             low = beta;
  67         }
  68         else
  69         {
  70             high = beta;
  71         }
  72     }
  73     return beta;
  74 }
  75
  76 static real compute_lj_function(real beta, real rc)
  77 {
  78     real xrc, xrc2, xrc4, result;
  79     xrc    = beta * rc;
  80     xrc2   = xrc * xrc;
  81     xrc4   = xrc2 * xrc2;
  82     result = std::exp(-xrc2) * (1 + xrc2 + xrc4 / 2.0);
  83
  84     return result;
  85 }
  86
  87 real calc_ewaldcoeff_lj(real rc, real rtol)
  88 {
  89     real beta = 5, low, high;
  90     int  n, i = 0;
  91
  92     do
  93     {
  94         i++;
  95         beta *= 2.0;
  96     } while (compute_lj_function(beta, rc) > rtol);
  97
  98     /* Do a binary search with tolerance 2^-60 */
  99     n    = i + 60;
 100     low  = 0;
 101     high = beta;
 102     for (i = 0; i < n; ++i)
 103     {
 104         beta = (low + high) / 2.0;
 105         if (compute_lj_function(beta, rc) > rtol)
 106         {
 107             low = beta;
 108         }
 109         else
 110         {
 111             high = beta;
 112         }
 113     }
 114     return beta;
 115 }