Ideal gas state calculation

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