The best way to calculate heat capacity is to differentiate energy with respect to temperature for CV ( extract energy and corresponding temperatures from g_energy) and perform numerical differentiation to obtain Cv<br><br>Jestin B. Mandumpal Ph.D (WABRI), MRSC<br>
Computational & Physical Chemist<br><br><br>From: Haiqing Zhao <haizhao@mtu.edu><br>Sent: Tue, 01 Nov 2011 21:42:38 <br>To: Discussion list for GROMACS development <gmx-developers@gromacs.org><br>Subject: [gmx-developers] details about Heat capacity caculation<br><br>
Dear Gmxers,<br>
<br>
I have one question about how Gromacs calculate the heat capacity. I dont think it is only Enthalpy fluctuation稫T^2) in NPT ensemble. And also I checked it in NVT, it is not simply Total energy(/internal energy) fluctuation稫T^2).<br>
<br>
One data for NPT is: ( -nmol 465 -nconstr 3 )<br>
<br>
Energy Average Err.Est. RMSD Tot-Drift<br>
-------------------------------------------------------------------------------<br>
Potential -44.6187 0.007 0.293318 0.0281861 (kJ/mol)<br>
Kinetic En. 7.59226 0.0032 0.19886 0.0201858 (kJ/mol)<br>
Total Energy -37.0264 0.0091 0.359776 0.0483719 (kJ/mol)<br>
Temperature 300.076 0.13 7.85971 0.797822 (K)<br>
Pressure 0.663728 0.16 447.392 -0.388627 (bar)<br>
Volume 14.3219 0.0093 0.133543 -0.0667073 (nm^3)<br>
pV 0.438734 9.5e-05 0.00136276 -0.000679764 (kJ/mol)<br>
Enthalpy -17216.8 4.2 167.296 22.4922 (kJ/mol)<br>
<br>
Temperature dependent fluctuation properties at T = 300.076. #constr/mol = 3<br>
Isothermal Compressibility: 3.00555e-05 /bar<br>
Adiabatic bulk modulus: 33271.8 bar<br>
Heat capacity at constant pressure Cp: 67.9219 J/mol K<br>
Thermal expansion coefficient alphaP: 9.66233e-05 1/K<br>
<br>
And data for NVT, ( -nmol 7631 -nconstr 3 )<br>
<br>
Energy Average Err.Est. RMSD Tot-Drift<br>
-------------------------------------------------------------------------------<br>
Total Energy -33.2264 0.0003 0.0900307 -0.000219237 (kJ/mol)<br>
Temperature 300.008 0.0063 1.95324 -0.0148441 (K)<br>
<br>
Temperature dependent fluctuation properties at T = 300.008. #constr/mol = 3<br>
Heat capacity at constant volume Cv: 70.1813 J/mol K<br>
<br>
(BTW, I found that (1)if you choose Enthalpy as output, g_energy will always give Cp, no matter what ensemble it is. (2)And only you choose tot energy and temp as output, no other terms, g_energy will give you Cv. If not, you cannot get Cv by g_energy.)<br>
<br>
I appreciate if anybody could help me to check it.<br>
Thanks!!<br>
<br>
---------------------------------------------<br>
Haiqing Zhao<br>
PH.D. candidate<br>
in Computational Biophysics<br>
Michigan Technological University<br>
http://www.phy.mtu.edu/~haizhao<br>
<br>
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