CHARMM force field potential energy surface

For each of these interactions, a model complex was designed the BSSE-corrected MP2 interaction potential energy surface (IPES) was calculated then the CHARMm IPES was validated against the MP2 one. If the CHARMm force field cannot produce a satisfactory IPES, the force field will be reparameterized using an automatic least square optimization program OPTMM written by G.D. in C language [24]. 

The success of any molecular simulation method relies on the potential energy function for the system of interest, also known as force fields [27]. In case of proteins, several (semi)empirical atomistic force fields have been developed over the years, of which ENCAD [28,29], AMBER [30], CHARMM [31], GRO-MOS [32], and OPLSAA [33] are the most well known. In principle, the force field should include the electronic structure, but for most except the smallest systems the calculation of the electronic structure is prohibitively expensive, even when using approximations such as density functional theory. Instead, most potential energy functions are (semi)empirical classical approximations of the Born-Oppenheimer energy surface. 

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