Core–valence correlation effects in the ground and low-lying excited states of GaN

Abstract

We present careful analysis of the accuracy of molecular properties of the GaN molecule calculated by the Coupled Cluster CCSD(T) method including the core–valence correlation effects. We demonstrate that calculations of the core–valence correlation effects with basis sets which were optimised considering just valence electrons of the gallium atom lead to unreliable results. Our objective is to find the relation between the accuracy and the oversimplification due to the use of aug-cc-pVXZ basis sets when 3d and eventually also 3s3p electrons are correlated with these basis sets, having in mind calculations of larger systems. We observe the oscillatory behaviour of molecular properties with increasing cardinal basis set number X in the aug-cc-pVXZ series. This is the main deficiency of this more economical basis set. Extrapolation to the complete basis set limit with this series is only possible starting with the aug-cc-pVQZ basis. Regular behaviour with increasing X show results with the aug-cc-pCVXZ series. Reasonable molecular properties are provided by the ANO-RCC basis sets with 3d ¹⁰4s ²4p ¹ correlated electrons of Ga.

Keywords:

• GaN molecular properties
• excited states
• core correlation effects
• CCSD(T), CASPT2, aug-cc-pVXZ and aug-cc-PCVXZ basis sets

Acknowledgements

This work was supported by the Slovak Research and Development Agency, contracts No. APVV-20-018405, APVV-LPP-0155-09, and the Slovak Grant Agency VEGA under the contract No. 1/0428/09. Support from EURATOM, contract No.FU07-CT-2006-00441 is also gratefully acknowledged.