Hello gmx users. I have tried to calculate the enthalpy of vaporization of toluene liquid, which is about 38kJ/mol (exp. value) using the OPLS-UA model (7 sites). I generated the toluene topology adapting a template provided by PRODRG. I have obtained acceptable value for density and structure of the liquid, but for the enthalpy the value is much lower than expected; 18kJ/mol.
I know there are subtleties in the treatment of aromatic systems. At Gmx users list I found some tips (see for example http://oldwww.gromacs.org/pipermail/gmx-developers/2009-January/002965.html), but still I can not solve the problem. I already tried numerous ways to circumvent this problem and none worked well. It seems to me the problem may lie in the definition of the dihedrals. Below is the topology that I have used and the main simulation parameters used. If someone can help me I would be very grateful. Bests eef **** toluene.itp **** [ moleculetype ] ; Name nrexcl TOL 3 [ atoms ] ; nr type resnr resid atom cgnr charge mass 1 opls_067 1 TOL CH3 1 0.000 15.0350 2 opls_076 1 TOL CB 1 0.000 12.0110 3 opls_075 1 TOL CD 1 0.000 13.0190 4 opls_075 1 TOL CD 1 0.000 13.0190 5 opls_075 1 TOL CD 1 0.000 13.0190 6 opls_075 1 TOL CD 1 0.000 13.0190 7 opls_075 1 TOL CD 1 0.000 13.0190 [ bonds ] ; ai aj fu c0, c1, ... 1 2 1 0.153 334720.0 0.153 334720.0 ; CH3 CB 2 3 1 0.141 418400.0 0.141 418400.0 ; CB CD 2 7 1 0.141 418400.0 0.141 418400.0 ; CB CD 3 4 1 0.141 418400.0 0.141 418400.0 ; CD CD 4 5 1 0.141 418400.0 0.141 418400.0 ; CD CD 5 6 1 0.141 418400.0 0.141 418400.0 ; CD CD 6 7 1 0.141 418400.0 0.141 418400.0 ; CD CD [ angles ] ; ai aj ak fu c0, c1, ... 1 2 3 1 120.0 418.4 120.0 418.4 ; CH3 CB CD 1 2 7 1 120.0 418.4 120.0 418.4 ; CH3 CB CD 3 2 7 1 120.0 418.4 120.0 418.4 ; CD CB CD 2 3 4 1 120.0 418.4 120.0 418.4 ; CB CD CD 3 4 5 1 120.0 418.4 120.0 418.4 ; CD CD CD 4 5 6 1 120.0 418.4 120.0 418.4 ; CD CD CD 5 6 7 1 120.0 418.4 120.0 418.4 ; CD CD CD 2 7 6 1 120.0 418.4 120.0 418.4 ; CB CD CD [ dihedrals ] ; ai aj ak al fu c0, c1, m, ... 2 7 3 1 2 0.0 1673.6 ; imp CB CD CD CH3 2 3 4 5 1 180.0 12.9704 2 ; imp CB CD CD CD 3 4 5 6 1 180.0 12.9704 2 ; imp CD CD CD CD 4 5 6 7 1 180.0 12.9704 2 ; imp CD CD CD CD 5 6 7 2 1 180.0 12.9704 2 ; imp CD CD CD CB 6 7 2 3 1 180.0 12.9704 2 ; imp CD CD CB CD 7 2 3 4 1 180.0 12.9704 2 ; imp CD CB CD CD **** toluene.mdp **** integrator = md nstlist = 5 ns_type = grid pbc = xyz rlist = 1.2 domain-decomposition = no coulombtype = Cut-off rcoulomb-switch = 0 rcoulomb = 1.2 vdw-type = Cut-off rvdw-switch = 0 rvdw = 1.2 DispCorr = EnerPres table-extension = 1 energygrp_table = fourierspacing = 0.12 Tcoupl = berendsen tc-grps = System tau_t = 0.1 ref_t = 300 Pcoupl = No Pcoupltype = isotropic tau_p = 1 compressibility = 5.0e-5 ref_p = 1.1 constraints = none constraint-algorithm = Lincs unconstrained-start = no energygrp_excl = **** toluene.gro **** PRODRG COORDS 7 1TOL CH3 1 0.159 -0.143 -0.050 1TOL CB 2 0.154 -0.227 0.078 1TOL CD 3 0.224 -0.349 0.084 1TOL CD 4 0.219 -0.426 0.203 1TOL CD 5 0.145 -0.379 0.313 1TOL CD 6 0.075 -0.256 0.308 1TOL CD 7 0.080 -0.180 0.189 0.51300 0.51300 0.51300 _______________________________________ Eudes Eterno Fileti Centro de Ciências Naturais e Humanas Universidade Federal do ABC — CCNH Av. dos Estados, 5001 Santo André - SP - Brasil CEP 09210-971 +55.11.4437-0196 http://fileti.ufabc.edu.br
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