[gmx-users] how to calculate the vaporation enthalpy of pure small organic compound

[Jinyao Wang]

 Hi gmx-users,
I want to  calcualted the the vaporation enthalpy of benzaldehyde 
 I think that the the vaporation enthalpy is the sum of non-bond interacion among the benzaldehyde molecule.    
So I made a NPT system simulation including 512 benzaldehyde molecule.  
The following is the intermolecule nonbond interaction using the g_energy.	

     LJ-(SR)= -21128.7 Kj/mol
     LJ-(LR)= -889.855 Kj/mol
     Coulomb-(SR)= -3884.6 Kj/mol
     Coul.-recip = -2261.51 Kj/mol
  the sum of non-bond interaction = LJ-(SR) + LJ-(LR) + Coulomb-(SR) + Coul.-recip = 28164.665 Kj/mol
   But the experimental value of he vaporation enthalpy = 50.3 Kj/mol
I have no ideal that why they have so much different. Now I don't know how to solve it.
Any suggestion will be appreciated.
 
 This following is my md.mdp file.

title               =  fws 
cpp                 =  /usr/bin/cpp 
constraints         =   all-bonds
integrator          =  md
dt                  =  0.002    ; ps !
nsteps              =  500000   ; total 1ns
nstcomm             =  1
nstxout             =  500
nstvout             =  0
nstfout             =  0
nstlist             =  10 
ns_type             =  grid
rlist               =  1.0
coulombtype         =  PME
rcoulomb            =  1.0
vdwtype             =  cut-off
rvdw                =  1.4
fourierspacing          =  0.12
fourier_nx              =  0
fourier_ny              =  0
fourier_nz              =  0
pme_order               =  6
ewald_rtol              =  1e-5
optimize_fft            =  yes
; Berendsen temperature coupling is on in three groups
Tcoupl              =  berendsen
tau_t               =  0.1
tc_grps             =  system
ref_t               =  298.15
; Pressure coupling is on
Pcoupl              =  berendsen
tau_p               =  2.0
compressibility     =  4.6e-5
ref_p               =  1.0
; Generate velocites is on at 323.15 K.
gen_vel             =  yes
gen_temp            =  298.15
gen_seed           =  173529 				

　　　　　　　
               　Jinyao Wang
　　　　　　　　wangjy at ciac.jl.cn
　　　　　　　　　　2009-10-23