# Enthalpy of vaporization calculation in CP2K

bharat bharats... at gmail.com
Tue Dec 9 14:44:53 UTC 2014

Hello Garold,
I was using experimental vapor density to calculate the box for 64 water
molecules instead of 1 water in a box used for liquid.

I did quick check, it seems I am getting reported value by following your
procedure. I am using PM3 as a reference calculation to make sure that I am
on the right path.

Thank you.

Best Regards,
Bharat

On Tuesday, December 9, 2014 9:07:09 AM UTC-5, garold wrote:
>
> Hi Bharat,
>
> We had some extensive email correspondence a few months ago when I gave
> you additional assistance, both input and output files and software
> changes. I hope that was helpful. Please let me know how it went.
>
> As for your current problem, I think you need to review some basics which
> can be found in any molecular dynamics book. You need to ask yourself,
> What is internal energy and how do I obtain it consistently from an MD
> code? Basically, internal energy U can be approximated as E_int/n where
> E_int is the interaction energy obtained from differences of potential
> energies. For a liquid with n molecules, with a DFT (or semiempirical)
> simulation you may calculate this as:
>
> U = [ (Average potential energy of n waters from the liquid simulation under
> some thermodynamic conditions) - n*(Potential energy of 1 water in the
> same box size, with same cutoffs, etc.) ] / n
>
> The assumption here is of course that the molecules in the gas phase are
> quite diffuse and barely interact with one another (this is why you use 1
> molecule). You can see my earlier paper (or other similar papers) on
> water clusters for additional discussion: J. Chem. Phys. 132, 164102
> (2010) , in particular page 164102-10 and also Figure 2 to see how
> E_int/n of water clusters start to approach the experimental liquid
> internal energy as the cluster size gets large, provided the correct
> Hamiltonian is used. Another assumption is that the vibrations and thus
> kinetic energies will be similar in the liquid and in the gas (this allows
> you to use the potential energies rather than the total energies). Of
> course, as also mentioned by the other respondents, with the PM3 Hamiltonian
> you will not obtain the experimental result but the exercise is still
> useful for you to see that you understand what you are doing.
>
> Are you you doing this or are you doing something else?
>
> Best,
> Garold
>
>
> On Monday, December 8, 2014 7:58:54 PM UTC+2, Jano... at googlemail.com
> wrote:
>>
>> Dear Bharat,
>>
>> I am not convinced about the usage of semiempericals like PM3 for such a
>> purpose, but it is your business...
>> 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
>> test your approximation:
>> 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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