Hartree-Fock model time-dependent

Before we do so it is worth-while to establish some conventions and terminology in this area. The obvious name for a model of electronic structure which has a time-dependent Hamiltonian and consists of a single determinant of orbit s and remains a single determinant at all times is the Time-Dependent Hartree-Fock (TDHF) model, and this is the terminology which will be used here. However, there is, particularly in the theoretical physics literature, another related usage. Because the use of perturbation theory is so much their stock-in-trade, many theoretical physicists use the term time-dependent Hartree-Fock to mean the first-order (in the sense of perturbation theory) approximation to what we will call the time-dependent Hartree-Fock model. 

Currently the time dependent DFT methods are becoming popular among the workers in the area of molecular modelling of TMCs. A comprehensive review of this area is recently given by renown workers in this field [116]. From this review one can clearly see [117] that the equations used for the density evolution in time are formally equivalent to those known in the time dependent Hartree-Fock (TDHF) theory [118-120] or in its equivalent - the random phase approximation (RPA) both well known for more than three quarters of a century (more recent references can be found in [36,121,122]). This allows to use the analysis performed for one of these equivalent theories to understand the features of others. 

Fitting of model potentials to supermolecular interactions like in Eq. (3-1) has its disadvantages the calculations have to be repeated many times and a predefined analytical expression of the model potentials in terms of atomic parameters is required. These parameters are not easily transferred to other situations. The internal state of, e.g., system A depends on the presence of another system X, and this will be different near system Y. Furthermore, the chosen analytical form of the potential may give rise to problems and errors. For example, Hartree-Fock (HF) calculations fitted to a power series in 1/r suggest that the 1/r6 terms have to do with dispersion, which is not part of the HF energy. Finally, such empirical potentials are best for describing situations close to those to which the parameters are fitted. Whenever the situation is very different from that, the results will be doubtful. 

In the following the polarizability and the first and second hyperpolarizabilities for urea calculated at the SCF level in vacuo and in water are reported. Both static and frequency dependent nonlinear properties have been calculated, with the Coupled Perturbed Hartree-Fock (CPHF) and Time Dependent-CPHF procedures that have been described above. The solvent model is the Polarizable Continuum Model (PCM) whereas vibrational averaging of the optical properties along the C-0 stretching coordinate has been obtained by the DiNa package both in vacuo and in solution. 

As in the Hartree-Fock molecular orbital theory, which is based on the independent particle model, the above Hartree product method also lacks enough correlation among the orbitals, and thereby the resultant accuracy is limited. To overcome the drawback, one can take account of the interaction among possible configurations (or the Hartree products) as in the configuration interaction method and multiconfiguration SCF methods in electronic structure theory. The multiconfigulational time-dependent Hartree... 

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