Atomic point charge

If separate D-RESP charge sets are fitted for every single one of the 36 frames, the standard deviation of the electrostatic field generated varies between 3.5 and 5% with respect to the full quantum reference. This accuracy is the best (in the least-squares sense) that can be obtained if the system is modeled with time-dependent atomic point-charges and represents the accuracy limit for a fluctuating point charge model of the dipeptide. 

Tab. 1.3 Comparison between different sets of atomic point charges for a zwitterionic Gly-Ala dipeptide in aqueous solution. D-RESP electrostatic potential derived charges [12] fitted to all 36 configurations. Hirshfeld average value of the Hirshfeld charges [89c] along the full trajectory, Amber AMBER 1995 force field [86], Gromos GROMOS96 force field [85], The charges of equivalent atoms are imposed to be equal.

Atom-atom point-charge schemes are expert systems that work on careful parameterization, witness the good performance in reproducing heats of sublimation (Fig. 8), where the calculation of one lattice energy takes a small fraction of a second. The atom-atom method cannot, however, be reconciled with the actual physics of the interaction. 

Fig. 6 The energy profile for a cyclic double hydrogen bond in acetic acid. Filled squares, total energy, open squares, atom-atom Coulombic energy, triangles. correct Coulombic energy. The result is insensitive to the choice of atomic point charges according to common schemes (e.g., Muliiken, ESP, etc.)...

Note that in Eqs. (2.22) and (2.23) the dielectric constant e is subscripted. Although one might expect the best dielectric constant to be that for the permittivity of free space, such an assumption is not necessarily consistent with the approximations introduced by the use of atomic point charges. Instead, the dielectric constant must be viewed as a parameter of the model, and it is moreover a parameter that can take on multiple values. For use in Eq. (2.22),... 

In QM/MM approaches, the MM system is represented through atomic point charges and (if a polarizable force field is used) atomic or molecular polarizabilities at selected points in the solvent molecules, we thus have... 

PCM and MPE calculations for the list of molecules given above. The agreement is especially good when net atomic point charges (gas-phase class IV CM2 [90] values) are used instead of doing a full SCF calculation. This is easily explained by the fact that the external electronic density plays a different role in both methods. When point charges are used, the whole charge distribution is inside the cavity and both methods are formally equivalent. 

The charge distribution of the solvent is represented by the atomic point charges (zs), hence the electrostatic operator is given by... 

Empirical force field calculations (MM2(8S)) using atomic point charges calculated by AMI calculations (MM2 // AMI) correctly reproduce the AMI surface for heterocyclic betaines 55, 126, and 127 (93JST105). The methodology allows extensive conformational analysis of medium to large-size molecules by semiempirical calculations (AMI). The interaction energies for the dimerization of these betaines have also been well reproduced. 

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