HDVV model

HDVV model Heisenberg-Dirac-Van Vleck model... 

The Heisenberg-Dirac-van Vleck (HDVV) spin Hamiltonian [1] is a well-known phenomenological model that became very popular and routinely used in the last... 

To be distinguished, the use of HDVV paradigm in systems with firm covalent bonds may really reveal the extension to which the model itself is applicable and to suggest the further necessary terms. This is indeed the case encountered in our approach. We found that for an improved description of selected set of states, the spin Hamiltonian has to be expanded with new terms, proposed here as the intercentric generalization of the traditional biquadratic terms. We propose and use here the following extended form ... 

The spin singlet states of HDVV Hamiltonian are those forming the problem of PJT effect related to the bond alternation. Using the symmetry labels of D4h one may write the following model Hamiltonian whose non-diagonal matrix element... 

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