Genre: Information/Explanation

11 Total energy in the DRF model The total energy in the DRF model is may be partitioned in various contributions : energy of the quantum system This is the energy of the quantum-mechanical system , as calculated with the supported wave functions . Any change in energy upon interaction with the classical system is to be measured against a vacuum calculation on the quantum-mechanical system . energy of the classical system This is the energy of the classical system , calculated as if there were no QM system present . The zero of energy depends on the specification of the classical system ; for example , atoms may be defined to be part of molecules , excluding their interactions , making the infinitely separate molecules defining the zero of energy , rather than infinitely separate atoms . In this energy interactions between classical subsystems ( e.g. molecules ) are included . They may be electrostatic , dispersion , repulsion , and induction interactions . interaction energy This is the sum of all separable interaction energies : electrostatic interactions : interactions between point charges in the classical system and the QM charge distribution ( nuclei and electrons ) . Induction Interactions , also called screening . This is the interaction of one subsystem with the reaction field induced by another subsystem . The interaction of the subsystem with its own reaction field is also part of the interaction , and is used to calculate the polarization energy , which is half the energy gain from induction at equilibrium . dispersion interactions : an estimate of the dispersion energy between QM and classical subsystems , based upon the Second-order Perturbation ( SOP ) expression for the dispersion interaction , may be calculated ( see also section 12.4 ) . model repulsion energy : a molecular mechanics force-field expression from CHARMM [ 7 ] is used to model ( Pauli ) repulsion between subsystems . NOTE : Electrons do not ' feel ' the repulsion and care has to be taken to avoid close contacts , which may lead to electrons ' wandering off ' the QM system . Devices to lessen this effect are available ( see section 12.2 ) Care should be taken in comparing total energies that the proper reference systems have been defined . In a single calculation , one does not have access to the polarization energy of the QM system : this is the change of the expectation value of the vacuum hamiltonian of the QM distribution ( and therefore internal QM energy ) upon interaction with the surroundings . A separate calculation in the absence of the surroundings ( or with the effect of the surroundings treated as a perturbation only ) is necessary to obtain this ( interaction ) energy . The definition of the zero of energy for the classical system has been discussed above . The total energy of QM + classical system is given on output under : configuration total energy . Contributions have been described in the Output section