# Ideal gas state calculation

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1. Oct 12, 2017

### hosein

Dear all,
I want to calculate thermodynamical properties of my molecule which I am calculating its thermodynamical properties of non-ideal part using Molecular dynamics. I need ideal gas state total energy, Cp, and Cv in several different temperatures. I am using opt+freq at B3lyp/6-311++G(d,p) setting to calculate thermochemistry properties. I am using gaussian 09, and I have some question, and I will be really grateful for your answers:
1- I would control temperature, but do you think I should change the pressure from its default (=1) to zero(or near zero) because of enforcing ideal situation, or it is not important in Ab initio simulation?
2- I need energy, Cp, and Cv. Thermochemistry part of frequency has total thermal energy, Cp, and Cv. Do you think I should use this total energy as my molecule calculations ideal part or the total energy in HF at the near of the end of output file of gaussian? How about Cp and Cv? If the energy should not be taken from thermochemistry part(why?) how can I use Cp and Cv there?

Best regards

2. Oct 18, 2017

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

3. Oct 19, 2017

### HAYAO

I'm not familiar with calculation of thermodynamic properties using Gaussian, but here's my try from common sense. Also your wording is a bit confusing, so sorry if I am completely misunderstanding your question.

1) Gaussian calculates the energies and heat capacities of the molecule as an ideal gas so it shouldn't matter. All you have to do is calculate the frequency of the molecule, and you'll get CV. You can easily convert that into CP for ideal gas by CP = R + CV.

2) From the context of your post, you are not trying to calculate the energy of the entire system. You are trying to calculate the internal thermal energy arising from molecular vibration. So you should use internal thermal energy "E (Thermal)" and volume-constant heat capacity "CV" that is in the table at near the end of the log file.

Reference: http://gaussian.com/thermo/

HAYAO

Last edited: Oct 19, 2017
4. Oct 19, 2017

### TeethWhitener

I was about to post this exact website. It's been a lifesaver for me doing thermo in Gaussian in the past.