# How to understand the correspondence between practical and theoretical free energy computation?

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 1:46:30 AM PST - Fri, Nov 15th 2013 Hi, I am trying to understand free energy calculation in NWChem. In NWChem documentation the free energy difference between two states is approximated as a sum of internal QM contribution and solvation free energy. I called this as 'practical free energy computation'. $\Delta W_{AB} \approx \Delta W_{AB}^{int} + \Delta W_{AB}^{solv}$ and $\Delta W_{AB}^{int} = E_{B}^{int} - E_{A}^{int}$ However, in the paper JCP-2007.127.051102 written by Marat et.al, proposed a thermodynamic cycle to perform free energy calculation. I called this as 'theoretical free energy computation'. $\Delta W_{AB} = ( \Delta W_{AA}^{DFT-ESP} - \Delta W_{BB}^{DFT-ESP} ) + \Delta W_{AB}^{ESP}$ According to my understanding, $\Delta W_{AB}^{solv}$ should correspond to $\Delta W_{AB}^{ESP}$. But, how to construct the correspondence between $\Delta W_{AB}^{int}$ and $( \Delta W_{AA}^{DFT-ESP} - \Delta W_{BB}^{DFT-ESP} )$ ? Is there some approximation between these? Any suggestion is appreciated. Thanks. Jingbo Edited On 2:06:12 AM PST - Fri, Nov 15th 2013 by Wjb0920