QM/MM methods for simulation of condensed

COMBINED QM/MM METHODS FOR THE SIMULATION OF CONDENSED PHASE PROCESSES USING AN APPROXIMATE DFT APPROACH... 

Mixed QM/MM Car-Parrinello simulation techniques are among the most powerful computational methods for exact numerical calculations of macroscopic and microscopic properties at finite temperatures of a large variety of condensed matter systems of current theoretical and experimental interest. Physical quantities that may be computed exactly and compared directly to experimental results, where available, include the kinetic, potential, and total energy, the radial distribution function, neutron scattering cross-sections, and so on. 

Any of the methods used in classical Monte Carlo and molecular dynamics simulations may be borrowed in the combined QM/MM approach. However, the use of a finite system in condensed phase simulations is always a severe approximation, even when appropriate periodic or stochastic boundary conditions are employed. A further complication is the use of potential function truncation schemes, particular in ionic aqueous solutions where the long-range Coulombic interactions are significant beyond the cutoff distance.Thus, it is alluring to embed a continuum reaction field model in the quantum mechanical calculations in addition to the explicit solute—solvent interaaions to include the dielectric effect beyond the cutoff distance. - uch an onion shell arrangement has been used in spherical systems, whereas Lee and Warshel introduced an innovative local reaction field method for evaluation of long-... 

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

In summary, the presented results demonstrate the capacity of combining IR-pump-probe methods with calculations on microsolvated base pairs to reveal information on hidden vibrational absorption bands. The simulation of real condensed phase dynamics of HBs, however, requires to take into account all intra- and intermolecular interactions mentioned in the Introduction. As far as DNA is concerned, Cho and coworkers have given an impressive account on the dynamics of the CO fingerprint modes [22-25]. Promising results for a single AU pair in deuterochloroform [21] have been reported recently using a QM/MM scheme [65]. 

Overall, current quantum chemical methods are adequate for many purposes. Improvements in speed and accuracy are ongoing. My personal view is that this does not represent the bottleneck in accurate predictive condensed phase simulations. If one accepts the idea that entirely quantum chemical simulations are not the optimal approach for most problems (and almost certainly not for biological problems), current methods perform well in the context of QM-MM modeling and in producing results for force field development. 

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