QUANTUM-MECHANICAL HARTREE-FOCK STUDY OF CALCITE (CACO3) AT VARIABLE-PRESSURE, AND COMPARISON WITH MAGNESITE (MGCO3)

The static crystal energy of calcite and its structure configuration a s functions of pressure were determined by ab initio all-electron peri odic Hartree-Fock calculations (CRYSTAL code). Ca, O and C atoms were represented by 22, 18 and 14 atomic orbitals, respectively, in form of contracted Gaussian-type functions. Comparison between theoretical an d experimental data was performed for binding energy, equilibrium unit -cell and bond lengths, bulk modulus and C33 and C-11 + C-12 elastic c onstants, and vibrational frequency of the symmetrical C-O stretching mode. The agreement is generally satisfactory. A larger compressibilit y is observed for structural parameters of calcite than for those of m agnesite coming from a similar calculation. The Ca-O and C-O chemical bonding was characterized by electron density maps and by Mulliken ato mic charges; these are discussed and compared to values determined by empirical fitting of Born-type interatomic potentials.