Molecular Configurations over Space

AMPAC. Version 6.0, 1997. 7204 Mullen, Shawnee, KS 66216. In this approach, a subset of electronic configurations is selected using perturbation theory fi-om the list generated by a full configuration interaction over a defined window of molecular orbitals (active space). 

The simulation of a molecular system at a finite temperature requires the generation of a statistically significant set of points in phase space (so-called configurations), and the properties of a system can be obtained as averages over these points. For a many-particle system, the averaging usually involves integration over many degrees of freedom. 

Fig. 3.11. Each of the two images contains superimposed configurations of a chain at many different instants in time in a molecular-dynamics simulation of a melt of such chains in a box. Over the time scale simulated, each chain appears to be confined to a tubelike region of space, except at the chain ends. [Reprinted by permission from K. Kremer and G. S. Grest, J. Chem. Phys., 92, 5057 (Copyright 1990 American Institute of Physics).

ASEP/MD, acronym for average solvent electrostatic potential obtained from molecular dynamics data, is a sequential QM/MM method that makes extensive use of the mean field approximation (MFA) [24], In solution, any static property A of the system must be calculated by averaging over the configurational space A defined by all the configurations thermally accessible to the system ... 

This model takes a more fundamental approach by regarding a molecule as a collection of valence electrons and positive cores. Just as the nature of atomic orbitals derives from the spherical symmetry of the atom, so will the properties of these new molecular orbitals be controlled by the interaction of the valence electrons with the multiple positive centers of these atomic cores. These new orbitals, unlike those of the hybrid model, are delocalized that is, they do not belong to any one atom but extend over the entire region of space that encompasses the bonded atoms. The available (valence) electrons then fill these orbitals from the lowest to the highest, very much as in the Aufbau principle that you learned for working out atomic electron configurations. For small mole-... 

Statistical thermodynamics of the electric double layer starts with modelling the electrolyte and the Interface. This can be done by specifying all inter-molecular and external Interactions in the phase space as a Hamiltonian. The notion of phase space was defined in sec. 1.3.9a and the Hamiltonian H was introduced in [1.3.9.11. As the kinetic part of the Hamiltonian does not contribute to the configuration Integrals, we sum only over the potential energies of the ions. In the Inhomogeneous system it Is customary to separate the interactions with the charged wall (the external" field) from the interlonlc ones. 

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