Heat of Mixing calculation

 Hi,

 

I am fairly new to molecular modeling so I am trying to do some basic things to validate my thinking. I am trying to test the calculation of the heat of mixing for common mixtures. I am calculating it from cohesive energy densities (CEDs) of the pure components and of mixtures. The pure components come out OK - CED is close to heat of vaporization. Unfortunately, I am coming up with some strange results for the heat of mixing. I cannot even correctly predict the sign of the delta H, much less magnitude!

Heat of Mixing Test
pure components:Mwmass fracmol frac of 1densityCED (J/cc)CED predicted (from deltaHvap)CED error (%)
ethanol46110.776576620.007945469
water18110.9521872173-0.006351462
phenol94111.05570645.86170210.116964982
benzoic acid122111.18793737.1007371-0.070490874
mixtures:mass fracmol frac of 1densitypred. densityCED (J/cc)CED ideal mixdeltaH/V (J/cc)deltaH J / molExpected deltaH J/mol
ethanol+water0.50.281250.830.8505813951209.1775671309.218701100.04113201.316-660
benzoic acid + phenol0.50.4351851.131.111210762695.79618686.5848216-9.21136-994.8271000


 

Simulation details: Forcite, unit cells are about 100 molecules, ultra-fine simulations, ultra-fine CED calculations. 300K. 200 ps of dynamcs. TP ensemble.