Full potential energy curve for N2 by the reduced multireference coupled-cluster method

Area of Science:

• Quantum Chemistry
• Computational Physics
• Molecular Spectroscopy

Background:

• Accurate potential energy curves are crucial for understanding molecular behavior.
• Nitrogen molecule (N2) is a fundamental system for benchmarking computational methods.
• Previous studies have relied on experimental data or less sophisticated theoretical models.

Purpose of the Study:

• To compute the full potential energy curves for the nitrogen molecule.
• To assess the accuracy of the reduced multireference coupled-cluster (RMR CC) methods.
• To compare theoretical results with experimental data and other computational approaches.

Main Methods:

• Employed the reduced multireference coupled-cluster method with singles and doubles (RMR CCSD) and its perturbatively corrected version for triples (RMR CCSD(T)).
• Utilized a 56-dimensional reference space and correlation-consistent, polarized, valence-quadruple-zeta (cc-pVQZ) basis sets.
• Compared computed potential energy curves and vibrational term values with experimental data and single-reference coupled-cluster methods.

Main Results:

• RMR CCSD demonstrated excellent performance, with systematic improvement as the reference space dimension increased.
• Computed RMR CCSD/cc-pVQZ values showed an average deviation of 8 cm⁻¹ for the first 19 vibrationally excited levels compared to experimental bands.
• Extrapolated RMR CCSD(T) results to the basis set limit yielded a deviation of 61 cm⁻¹.

Conclusions:

• The RMR CCSD and RMR CCSD(T) methods provide accurate potential energy curves for the nitrogen molecule.
• The computational approach is validated against experiment-based analytic potentials.
• These findings highlight the capability of RMR CC methods for accurate molecular electronic structure calculations.