Geometry optimizations with spinor-based relativistic coupled-cluster theory.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Relativistic quantum mechanics

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Including spin-orbit coupling (SOC) is crucial for heavy elements but computationally demanding.
• Efficient methods for geometry optimization with SOC are needed.

Purpose of the Study:

• To develop analytic gradients for relativistic coupled-cluster singles and doubles augmented with a non-iterative triples [CCSD(T)] method.
• To enable efficient geometry optimizations that include spin-orbit coupling effects within the molecular orbitals.
• To demonstrate the applicability of the new method for benchmark calculations.

Main Methods:

• Development of analytic gradients for the X2CAMF-CCSD(T) method.
• Utilizing an all-electron exact two-component Hamiltonian with atomic mean-field spin-orbit integrals (X2CAMF).
• Performing benchmark calculations for methyl halides (CH3X, X = Br, I, At) and RaSH+.

Main Results:

• Successful implementation of analytic gradients for X2CAMF-CCSD(T).
• Efficient geometry optimizations with SOC-included orbitals are now possible.
• Benchmark calculations provided accurate equilibrium structures, harmonic vibrational frequencies, rotational constants, and infrared spectra.

Conclusions:

• The developed X2CAMF-CCSD(T) method provides an efficient and accurate approach for electronic structure calculations involving spin-orbit coupling.
• This method is applicable to systems with heavy elements and radioactive species, aiding spectroscopic studies.
• The implementation facilitates precise predictions of molecular properties relevant to chemical research.