[gmx-users] TIP3P versus SPC?

[Michael Shirts]

Hi, all - I was just wondering about the differences of implementation of
TIP3P and SPC in GROMACS.  I get a couple of odd things - below are outputs of
g_energy from two simulations, identical conditions except that one has TIP3P
and the other has SPC.  PME, rlist = rcoulomb = 1.15, Switched VdW 0.9-1.1,
dispersion correction, 900 molecules, Nose-Hoover, Parrinello-Rahman.

The densities remain close to the experimental values, but the coulombic
energy for TIP3P is 20x that of SPC. Do you have any idea why this is the
case?  It just seems rather odd that this differece in coulombic energy should
exist without there being other large differences in other properties. . .

Thanks,
Michael Shirts
Pande Group
Chemistry Department
Stanford University

SPC with PME
LJ (SR)                     6464.53    259.014    259.007 0.00642346    6.42347
Disper. corr.              -162.668    1.73525    1.73521 -3.73529e-05 -0.0373529
Coulomb (SR)               -41653.8    560.793    560.781 -0.0128294   -12.8294
Coulomb (LR)               -2074.37    11.5334    11.5334 0.000108335   0.108336
Potential                  -37426.3    386.944    386.939 -0.00633505   -6.33507
Kinetic En.                 6690.03    208.658    208.657 -0.00205789    -2.0579
Total Energy               -30736.3     542.66    542.655 -0.0083929   -8.39292
Temperature                 298.008    9.29468    9.29464 -9.16688e-05  -0.091669
Pressure (bar)              2.36887    478.957    478.956 0.00256479    2.56479
Box-X                       3.02334    0.01077  0.0107698 -2.24437e-07 -0.000224437
Volume                      27.6361   0.295627   0.295621 -6.05586e-06 -0.00605588
Density (SI)                974.335    10.3936    10.3934 0.000223733   0.223733
Heat Capacity Cv:        12.49 J/mol K (factor = 0.00097278)
Isothermal Compressibility: 7.68593e-05 /bar
Adiabatic bulk modulus:        13010.8  bar

TIP3P with PME
LJ (SR)                      5566.8     248.61    248.593  0.0111267    9.92947
Disper. corr.              -155.928    1.80238    1.80221 -9.61068e-05 -0.0857659
Coulomb (SR)                -606615    584.342     584.33 -0.0142947   -12.7567
Coulomb (LR)                -199772    11.8148    11.8148 3.28827e-05  0.0293446
Potential                   -800976     419.16    419.159 -0.00323129   -2.88361
Kinetic En.                 6690.07    224.102    224.099 -0.00426235   -3.80373
Total Energy                -794286    595.936    595.933 -0.00749363   -6.68733
Temperature                 298.009    9.98263    9.98251 -0.000189867  -0.169437
Pressure (bar)              2.35245    467.995    467.966   0.020043    17.8864
Box-X                       3.01574  0.0116386  0.0116374 -6.4277e-07 -0.000573609
Volume                      27.4285   0.317837   0.317804 -1.78581e-05 -0.0159366
Density (SI)                 981.73    11.3478    11.3468 0.000605091   0.539984
Heat Capacity Cv:      12.4928 J/mol K (factor = 0.0011221)
Isothermal Compressibility: 8.95143e-05 /bar
Adiabatic bulk modulus:        11171.4  bar

Cheers,
Michael