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lammps/tools/fep/bar.py
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bar.py
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#!/usr/bin/env python
# bar.py - Bennet's acceptance ratio method for free energy calculation
import sys, math
if len(sys.argv) < 4:
print "Bennet's acceptance ratio method"
print "usage: bar.py temperature datafile01 datafile10 [delf_lo delf_hi]"
print " datafile01 contains (U_1 - U_0)_0 in 2nd column"
print " datafile10 contains (U_0 - U_1)_1 in 2nd column"
print " (first column is index, time step, etc. and is ignored)"
print " delf_lo and delf_hi are optional guesses bracketing the solution"
sys.exit()
if len(sys.argv) == 6:
delf_lo = float(sys.argv[4])
delf_hi = float(sys.argv[5])
else:
delf_lo = -50.0
delf_hi = 50.0
def fermi(x):
if x > 100:
return 0.0
else:
return 1.0 / (1.0 + math.exp(x))
def avefermi(eng, delf):
ave = 0.0
n = 0
for du in eng:
ave += fermi((du + delf) / rt)
n += 1
return ave / n
def bareq(delf):
ave0 = avefermi(eng01, -delf)
ave1 = avefermi(eng10, delf)
return ave1 - ave0
def bisect(func, xlo, xhi, xtol = 1.0e-4, maxit = 20):
if xlo > xhi:
aux = xhi
xhi = xlo
xlo = aux
if func(xlo) * func(xhi) > 0.0:
print("error: root not bracketed by interval")
sys.exit(2)
for i in range(maxit):
sys.stdout.write('.')
sys.stdout.flush()
xmid = (xlo + xhi) / 2.0
if func(xlo) * func(xmid) < 0.0:
xhi = xmid
else:
xlo = xmid
if xhi - xlo < xtol:
return xmid
return xmid
print "Bennet's acceptance ratio method"
print sys.argv[1], " K"
rt = 0.008314 / 4.184 * float(sys.argv[1])
eng01 = [] # read datafiles
with open(sys.argv[2], 'r') as f:
for line in f:
if line.startswith('#'):
continue
eng01.append(float(line.split()[1]))
eng10 = []
with open(sys.argv[3], 'r') as f:
for line in f:
if line.startswith('#'):
continue
eng10.append(float(line.split()[1]))
sys.stdout.write("solving")
delf = bisect(bareq, delf_lo, delf_hi)
print "."
ave0 = avefermi(eng01, -delf)
ave1 = avefermi(eng10, delf)
print "<...>0 = ", ave0
print "<...>1 = ", ave1
print "deltaA = ", delf
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