# [lammps-users] heat capasity at constant pressure for water

Hi,

I know that heat capacity at constant volume(cv) and constant pressure(cp) can be calculated based on fluctuations of energy and enthalpy(Etotal+P*V) respectively. Both cv and cp for water are measured to be 4.18 KJ / Kg mol at room temp. Using LAMMPS, I obtained cv=4.3 (in NVT ensemble) and cp=44.5 (in NPT ensemble) for water(liquid) at room temp. Hence, I’m able to reproduce exact cv, but not cp. I suspect the reason I’m not able to do it for cp is that the pressure fluctuates very large. No matter how I tune the damping parameters for pressure, Pdamp, from 100 fs to 10 000 fs (in real units), I got pressure fluctuates at plus-minus 1000 atm. After that, I applied drag from 0.2 to 2.0, and I got the same results. Can someone please guide me?
Send
Thank you.

Regards,
Christopher

Hi,

I know that heat capacity at constant volume(cv) and constant pressure(cp) can be calculated based on fluctuations of energy and enthalpy(Etotal+P*V) respectively. Both cv and cp for water are measured to be 4.18 KJ / Kg mol at room temp. Using LAMMPS, I obtained cv=4.3 (in NVT ensemble) and cp=44.5 (in NPT ensemble) for water(liquid) at room temp. Hence, I’m able to reproduce exact cv, but not cp. I suspect the reason I’m not able to do it for cp is that the pressure fluctuates very large. No matter how I tune the damping parameters for pressure, Pdamp, from 100 fs to 10 000 fs (in real units), I got pressure fluctuates at plus-minus 1000 atm. After that, I applied drag from 0.2 to 2.0, and I got the same results. Can someone please guide me?
Send
Thank you.

Regards,
Christopher

How big is your system? Pressure fluctuations are typically large,
but should get smaller for a bigger system. Aidan may want
to comment on this approach to Cp.

Steve