# [gmx-users] Re: Re: how to calculate the vaporation enthalpy of pure small organic compound (David van der Spoel)

Jinyao Wang wangjy at ciac.jl.cn
Sat Oct 24 03:51:11 CEST 2009

```Hi David,
Thank you for your help. I am very sorry to trouble you.
I have know your means that the gas-phase energy should be considered.
But I am confused that you said "the gas-phase energy (with com removed) and add kT".
What is "the gas-phase energy (with com removed) " and Could you talk about the details?

>>
>> Jinyao Wang wrote:
>>>  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.
>>>
>>
>> These are totals for the intermolecular energy of the system.  I'm
>> guessing you didn't use "-nmol 512" to calculate these results?
>
>And then subtract the gas-phase energy (with com removed) and add kT.

Jinyao Wang
wangjy at ciac.jl.cn
2009-10-24
```