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,

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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