Tokarev V. Quantum-chemical simulation of magnetic properties of quasionedimensional magnets based on transition metal compounds

The thesis is devoted to simulation of magnetic properties of quasi-one-dimensional transition metal complex compounds using effective hamiltonians in quantum-chemical valence bond method. The scope of this work also covers development and applicability checking of approximate methods for low-temperature magnetic properties simulation and calculation of lower energy spectrum of systems of this kind. For infinite repulsion Hubbard model defined on square lattice stripes with finite width n with alternation of single-electron energies αi of neighboring unit cells ground state and lower excitations were studied using degenerate perturbation theory and numerical calculations using exact diagonalization method. With help of spin permutation technique and second-order degenerate perturbation theory the low-energy effective Hamiltonians were derived for different electron densities and alternation strengths. It was shown that electron density gives maximal value of ground state total spin in limit of weak unit cell interactions. Moreover, it was shown that for this type of lattice electron filling corresponds to greater stability of maximal value of ground state spin regarding to increase of unit cells’ interactions compared to systems without alternation of one-electron energies. For 2-leg ladder without orbital energy alternation it was shown that ground state cannot have maximal spin. Also, using numerical method of exact diagonalization of finite lattice Hamiltonian for width-3 lattice with ρ it was demonstrated that decrease of electronic density accompanied with single-electron energy alternation can destroy ground state with maximal value of total spin. For density ρ presence ofαi alternation increases stability of maximumspin ground state regarding to small variations of αi.