New Ab Initio Potential Energy Surfaces for NH₃ Constructed from Explicitly Correlated Coupled-Cluster Methods

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Molecular Spectroscopy

Background:

• Accurate potential energy surfaces (PESs) for ammonia (NH3) are crucial for understanding its properties.
• Standard computational methods like coupled cluster with singles and doubles, and perturbative triples (CCSD(T)) struggle with basis set convergence for NH3 PESs.
• Existing high-accuracy NH3 PESs often rely on empirical refinements, limiting their predictive power.

Purpose of the Study:

• To develop an accurate, purely *ab initio* potential energy surface for the NH3 molecule.
• To investigate the convergence of CCSD(T) calculations with respect to basis set size for NH3.
• To improve the accuracy of calculated spectroscopic properties compared to experimental data.

Main Methods:

• Extrapolation of coupled cluster with singles and doubles, and perturbative triples (CCSD(T)) energies to the complete basis set limit using augmented correlation-consistent polarized valence basis sets of increasing size (aug-cc-pCVXZ, X=T, Q, 5, 6).
• Comparison with explicitly correlated CCSD(T)-F12a calculations.
• Inclusion of corrections for one-electron relativistic effects, diagonal Born-Oppenheimer correction, and high-order electronic correlations (up to CCSDTQP).

Main Results:

• CCSD(T)/aug-cc-pCV6Z calculations alone are insufficient for describing the NH3 PES accurately across a wide range of configurations.
• Complete basis set extrapolation of CCSD(T) energies yields a PES closer to that obtained with the CCSD(T)-F12a method.
• The final 'pure' *ab initio* PES significantly reduces the root-mean-square deviation of calculated band centers compared to experimental data, achieving approximately half the error of the previous best *ab initio* PES.

Conclusions:

• A highly accurate, parameter-free *ab initio* PES for NH3 has been successfully constructed.
• The developed PES provides a substantial improvement in predicting spectroscopic properties, particularly band centers in the 0-7000 cm⁻¹ region.
• This work establishes a new benchmark for *ab initio* calculations of molecular PESs, demonstrating the importance of basis set extrapolation and high-order correlation treatments.