Theoretical dipole transition moments for transitions between bound electronic states and non-adiabatic coupling matrix elements between2Σ+of HeH

Spectroscopic properties of the low-lying states of the HeH radical

Four lowest electronic states (X2Σ+, A2Σ+, B2Π and C2Σ+) of the HeH radical are investigated by performing high-precision ab initio calculations. Potential energy curves of HeH are calculated using the internally contracted multireference configuration interaction method in combination with the basis set of aug-cc-pV6Z. To improve the accuracy, additional Rydberg basis functions and the Pople correction are included in the calculations. The spectral parameters are evaluated and compared with literature data. The spin-orbit coupling effect of B2Π is taken into account by using the Breit-Pauli operator. Transition dipole moments between those four electronic states are also derived to obtain their emission spectra and radiative lifetimes. Radiative and predissociation lifetimes of several vibrational levels are also calculated, including the bound-continuum and bound-bound systems. Especially, the predissociation lifetimes are elaborated in detail. The results are in good agreement with the reported ones. Additionally, the emission spectra of bound A2Σ+, B2Π and C2Σ+ states to continuum X2Σ+ state are simulated at different temperatures.

Multireference space without first solving the configuration interaction problem

We further develop an idea to generate a compact multireference space without first solving the configuration interaction problem previously proposed for the ground state (GS) (Glushkov, Chem. Phys. Lett. 1995, 244, 1). In the present contribution, our attention is focused on low-lying excited states (ESs) with the same symmetry as the GS which can be adequately described in terms of an high-spin open-shell formalism. Two references Møller-Plesset (MP) like perturbation theory for ESs is developed. It is based on: (1) a main reference configuration constructed from the parent molecular orbitals adjusted to a given ES and (2) secondary double excitation configuration built on the GS like orbitals determined by the Hartree-Fock equations subject to some orthogonality constraints. It is shown how to modify the MP zeroth-order Hamiltonian so that the reference configurations and corresponding excitations are eigenfunctions of it and are compatible with orthogonality conditions for the GS and ES. Intruder states appearance is also discussed. The proposed scheme is applied to the GS, ES, and excitation energies of small molecules to illustrate and calibrate our calculations.

Optimized effective potential method for individual low-lying excited states

This paper presents an optimized effective potential (OEP) approach based on density functional theory (DFT) for individual excited states that implements a simple method of taking the necessary orthogonality constraints into account. The amended Kohn-Sham (KS) equations for orbitals of excited states having the same symmetry as the ground one are proposed. Using a variational principle with some orthogonality constraints, the OEP equations determining a local exchange potential for excited states are derived. Specifically, local potentials are derived whose KS determinants minimize the total energies and are simultaneously orthogonal to the determinants for states of lower energies. The parametrized form of an effective DFT potential expressed as a direct mapping of the external potential is used to simplify the OEP integral equations. A performance of the presented method is examined by exchange-only calculations of excited state energies for simple atoms and molecules.