[gmx-users] Different TI free energy values in 4.0.7 and 4.5.3

[Yan Chai]

Dear Gromacs users,

  I got different free energy values by Thermodynamic Integration (TI) in
Gromacs 4.0.7 and 4.5.3.

 When I calculated the hydration free energy of Chloride ion by using
Thermodynamic Integration method, the results for the free energy of
un-charging the Chloride ion from q=-e to 0 in SPC water are 359+/-6 kJ/mol
in Gromacs 4.0.7 and 385 +/-6 kJ/mol in Gromacs 4.5.3, respectively.

  The literature value is 392 kJ/mol (J. Phys. Chem. 1996, 100, 1206-1215,
Table 6), which is close to the result in Gromacs 4.5.3 but not in 4.0.7.

  As a check, Gromacs 3.3.1 gave the result as 381+/-3 kJ/mol.

  I searched the mail list and the bugzilla of Gromacs but didn't find
explanation for this issue of version.

  Does anyone know the reason for the different TI free energy values in
those different version of Gromacs?

  Thanks!

    Yan
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