src/gromacs/math/calculate-ewald-splitting-coefficient.cpp

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  37 #include "gmxpre.h"
  38
  39 #include "calculate-ewald-splitting-coefficient.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     }
  56     while (std::erfc(beta*rc) > rtol);
  57
  58     /* Do a binary search with tolerance 2^-60 */
  59     n    = i+60;
  60     low  = 0;
  61     high = beta;
  62     for (i = 0; i < n; i++)
  63     {
  64         beta = (low+high)/2;
  65         if (std::erfc(beta*rc) > rtol)
  66         {
  67             low = beta;
  68         }
  69         else
  70         {
  71             high = beta;
  72         }
  73     }
  74     return beta;
  75 }
  76
  77 static real compute_lj_function(real beta, real rc)
  78 {
  79     real xrc, xrc2, xrc4, result;
  80     xrc    = beta*rc;
  81     xrc2   = xrc*xrc;
  82     xrc4   = xrc2*xrc2;
  83     result = std::exp(-xrc2)*(1 + xrc2 + xrc4/2.0);
  84
  85     return result;
  86 }
  87
  88 real calc_ewaldcoeff_lj(real rc, real rtol)
  89 {
  90     real beta = 5, low, high;
  91     int  n, i = 0;
  92
  93     do
  94     {
  95         i++;
  96         beta *= 2.0;
  97     }
  98     while (compute_lj_function(beta, rc) > rtol);
  99
 100     /* Do a binary search with tolerance 2^-60 */
 101     n    = i + 60;
 102     low  = 0;
 103     high = beta;
 104     for (i = 0; i < n; ++i)
 105     {
 106         beta = (low + high) / 2.0;
 107         if (compute_lj_function(beta, rc) > rtol)
 108         {
 109             low = beta;
 110         }
 111         else
 112         {
 113             high = beta;
 114         }
 115     }
 116     return beta;
 117 }