# Enthalpy of vaporization calculation in CP2K

Mon Dec 8 18:00:07 UTC 2014

```Dear Bharat,

I am not convinced about the usage of semiempericals like PM3 for such a
Concerning the approach:
Calculating internal energy is simply calculating the average of the total
energy.  But, as the average of the kinetic energy is defined on a given
temperature (I assume, you run NVT simulations), I usually calculate the
average of the potential energy. The contribution from kinetic energy
should cancel when you subtract the liquid phase value from the gas phase
value.

Both potential energy and total energy are reported in the *.ener file in
cp2k.

Otherwise do you get the right value by simply adding RT to dU? Just to
why don't you add d(pV), that you can calculate from experimental densities.

Janos

On Monday, December 8, 2014 3:43:56 PM UTC+1, bharat wrote:
>
> Hi Samuel,
> It's a semiempirical calculation. I does not have any functional forms
> like GGA or hybrid. Calculation is correct because I was able to reproduce
> other properties. Isn't internal energy "Total energy" in CP2K? if not how
> do I calculate internal energy from cp2k results?
>
> Thanks.
>
> Bharat
>
> On Monday, December 8, 2014 3:52:56 AM UTC-5, Samuel Andermatt wrote:
>>
>> You will need to post your input and output files. Do you do GGA or
>> hybrid calculations, how do you account for the vdW forces?
>>
>> On Friday, December 5, 2014 4:17:45 PM UTC+1, bharat wrote:
>>>
>>> Hello,
>>> This is friendly reminder. Any suggestions?
>>>
>>> Bharat
>>>
>>> On Wednesday, December 3, 2014 12:56:10 PM UTC-5, bharat wrote:
>>>>
>>>> Hello Experts,
>>>>
>>>> I am trying to reproduce Enthalpy of vaporization using PM3.
>>>> Here are my calculation:
>>>> (I took Total energy value from CP2K output file as an Internal energy,
>>>> average is calculated over the MD. Experimental density is used for the
>>>> constant volume for both liquid and vapor calculation). Am I taking correct
>>>> energy value for internal energy?
>>>>
>>>> U_vapor(Avg) = -20797.3 eV
>>>> U_liquid(Avg) = -20802.9 eV
>>>>
>>>> Delta_U = 5.6 eV
>>>> I divided with 64 (# of water molecules) and converted to kcal/mol
>>>> = 2.02 kcal/mol
>>>>
>>>> I got the half value (reported value is 4.00 kcal/mol in G. Murdachaew
>>>> et al. J. Phys. Chem. A 115, 6046 (2011)). Because of this value, I got
>>>> only ~2.60 kcal/mol (RT= ~0.60 kcal/mol) of enthalpy of vaporization
>>>> (Delta_H = delta_U + RT).
>>>>
>>>> Can anyone please tell me where I am doing wrong? Where is the factor 2
>>>> missing?
>>>>
>>>> Thank you.
>>>>
>>>> Bharat
>>>>
>>>>
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