The reliability of three popular semiempirical quantum chemical methods (AMI, PM3, MNDO) for the treatment of tautomeric equilibria is tested in a series of five-membered nitrogen heterocycles. The known flaw of M N D O to overestimate the stability of compounds with two or more adjacent pyridine-like lone pairs is also present in AMI and to a somewhat lesser extent in PM3. Tautomeric species d...

The doping reaction of truncated boron nitride and carbon nanotubes with aluminium atom wastheoretically investigated. The AM1, PM3, and PM6 semiempirical methods have been used toevaluate the thermochemistry of doping reactions of single walled boron nitride nanotubes andcarbon nanotubes. The enthalpy changes, Gibbs free energy changes, and entropy changes of studieddoping reactions were evalu...

The numerical accuracy of linear scaling semiempirical methods LocalSCF and MOZYME is analyzed in comparison to conventional matrix diagonalization with respect to a variety of molecular properties including conformational energy, dipole moment, atomic charges, and bond orders. Major semiempirical MNDO, AM1, PM3, and PM5 Hamiltonians were considered in the study. As the numerical tests demonstr...

Two new semiempirical methods employing a Pairwise Distance Directed Gaussian modification have been developed: PDDG/PM3 and PDDG/MNDO; they are easily implemented in existing software, and yield heats of formation for compounds containing C, H, N, and O atoms with significantly improved accuracy over the standard NDDO schemes, PM5, PM3, AM1, and MNDO. The PDDG/PM3 results for heats of formatio...

Semiempirical orthogonalization-corrected methods (OM1, OM2, and OM3) go beyond the standard MNDO model by explicitly including additional interactions into the Fock matrix in an approximate manner (Pauli repulsion, penetration effects, and core-valence interactions), which yields systematic improvements both for ground-state and excited-state properties. In this Article, we describe the underl...

For large reactive systems, the calculation of energies can be simplified by treating the active part with a high-level quantum mechanical ~QM! ~ab initio or density functional! approach and the environment with a less sophisticated semiempirical ~SE! approach, as an improvement over the widely used hybrid quantum mechanical/molecular mechanical ~QM/MM! methods. An example is the interaction be...

Regularities and systematic trends among the Stark widths of 18 Zr IV spectral lines obtained by modified semiempirical approach have been discussed. Also we compared those calculated Stark broadening parameters with estimates according to Cowley, Purić et al. and Purić and Šćepanović and checked the possibility to find some new estimates. It is demonstrated as well that the formula of Cowley (...