Sankar Prasad Bhattacharyya

Some aspects of new orthonormality-constrained orbital optimisation technique

Abstract The performance of a orthonormality-constrained orbital optimisation procedure has been investigated in its one-configuration limit and compared with that of the traditional method of Roothaan. An intrinsic superiority of the method is manifested in the achievement of smooth and stable optimisation even in pathological cases where Roothaans method fails to converge. A simple “scaling” technique, somewhat akin to the “damping” procedure of Saunders et al. is developed to enhance the rate of descent on the energy surface. The possibility of generating localized Hartree—Fock orbitals directly by using the new method has also been explored and successfully implemented.

On the direct determination of constrained pure state one-electron density matrices: Part I. A new theoretical model for closed-shell systems

A new method based on the penalty-function way of satisfying equality constraints is proposed for the determination of constrained pure state one-electron density matrices for closed-shell many-electron systems. The algorithm suggested can handle many constraints simultaneously. Certain interesting features of the proposed algorithm are discussed with numerical examples.

an average “hole-potential” method for studying properties of molecules in excited states: a test calculation on thiophosgene

Abstract An average hole-potential method (AHP) for studying molecular electronic structures and properties in excited states is suggested. The improved virtual orbitals (IVOs) prepared by this method are adapted to all possible transitions on the average. AHP calculations performed on the excited states of thiophosgene at the CNDO/2 level yield encouraging results.

Accelerated convergence in SCF calculations and the level shifting technique

Abstract The possibility of improving the convergence rate in SCF calculations by exploiting the essential arbitrariness of the diagonal matrix elements of the Fock operator formed in the MO basis is studied. It is shown that it is possible to accelerate convergence in many cases by adopting a different form of level shifting technique (called the reverse level shifting technique by us).

Semiempirical molecular orbital studies on the electronic structure of molecules in excited states: The INDO/2-VN— 1 potential model: Part I.

The performance of the VN—1 potential model at the INDO/2 level of approximation in the calculation of transition energy, singlet—triplet splitting, change in molecular structure, inversion barrier and electron-density distribution in the excited electronic states of a few simple carbonyls is analysed. The method turns out to be reasonably successful in many ways.

On the direct determination of constrained pure state one-electron density matrices: II. A modified algorithm and its applications

An algorithm recently proposed by us for the direct determination of pure state one-electron density matrices (P) under externally imposed constraints has been remodelled. The modified algorithm is applied to the construction of ground state potential energy curve of lithium hydride molecule usingknown values of dipole moments of LiH at various internuclear distances as theexternal constraint. The equilibrium internuclear distances (Rc) is seen to be unaffected by the constraint, but the force constant improves remarkably. The relevant features of the constrained density vis-a-vis those of the unconstrained one are analyzed revealing some of the improved features of the constrained density.

The sign-change argument revisited

Abstract Recent examples suggesting the inapplicability of the “sign-change” argument in perturbation theory are critically analysed. It is shown that the arguments leading to such a conclusion are not entirely justified.

Applications of a novel algorithm for the calculation ofMCSCF wavefunction: a look into possible avenues of convergence acceleration

The efficacy of a method based on the direct inversion in the iterative subspace (DIIS) in accelerating the approach to self consistency in the calculation of theMCSCF wavefunction using a novel algorithm developed earlier, is compared with that of a simple damping technique. Although the ‘damping’ turns out to be ineffective in the ‘quadratic region’, it accelerates remarkably in the rate of descent on the energy hypersurface in the early stages of the iterative process which leads to an impressive overall increase in the rate of approach to self consistency. TheDIIS based procedure turns out to be ineffective when coupled to the present method and is plagued by ill conditioning problems. Calculations are done to compute the equilibrium geometrical parameter, charge density on different atoms, and dipole moment of HNO molecule in the lowest1,3nπ* states at theINDO/2-MCSCF level.

The least-squares method for bound states: a re-examination

Abstract The performance of the least-squares method is compared with that of the usual (Rayleigh-Ritz) variation method (UVM) in a specific bound-state calculation. Their relative merits and demerits are carefully analysed with respect to the approximate calculation of expectation values of particular types of observables.

CNDO bonding parameters in transition metal atoms

The method of calculating CNDO bonding parameters developed recently is extended to transition metal atoms. It is shown that one of the approximations introduced earlier can also be deduced by a more complete treatment of the imbalance problem in CNDO-MO theory. The conventionally calibrated bonding parameters indirectly incorporate important contributions from two-particle interactions. The parameters developed are used to compute the coefficients of metal-to-ligand transfer of spin in many hexafluro metallate ions of transition metals. The results are compared with those obtained by conventional CNDO-MO calculation. Comparison of the computed bonding parameters with other available values is also made.