Overview of Developments in the MRCC Program System.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate quantum chemical calculations are crucial for understanding molecular properties.
• Existing methods often face computational limitations for large systems or high accuracy.
• The MRCC package provides a suite of tools for advanced electronic structure calculations.

Purpose of the Study:

• To present the general features and recent developments of the MRCC quantum chemistry program.
• To highlight advancements in coupled-cluster (CC) and density functional theory (DFT) methods within MRCC.
• To showcase cost-reduction techniques and new approximations for improved computational efficiency.

Main Methods:

• Implementation of open-ended coupled-cluster (CC) codes, including CCSD(T).
• Utilization of cost-reduction techniques like natural orbital (NO) and local NO (LNO) approximations.
• Development of efficient schemes for approaching the complete basis set limit and analytic gradient calculations.

Main Results:

• Demonstration of state-of-the-art CC methods and combined wave function theory-DFT approaches.
• Significant reduction in computational demands through implemented cost-saving approximations.
• Extension of LNO-CCSD(T) to open-shell systems and novel methods for excited-state calculations.

Conclusions:

• The MRCC package offers a versatile and efficient platform for high-accuracy quantum chemical calculations.
• Recent developments enhance the applicability of advanced methods to a wider range of chemical problems.
• The program's availability for academic and commercial use facilitates broader scientific research.