Towards Multireference Equivalents of the HEAT Thermochemical Protocol

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate prediction of molecular properties requires advanced electronic structure methods.
• Strong electron correlation poses a challenge for standard single-reference methods.
• Multireference (MR) methods are needed to capture complex correlation effects.

Purpose of the Study:

• Evaluate internally contracted multireference coupled cluster (icMRCC) performance within the HEAT protocol.
• Assess icMRCC as an alternative to single-reference methods for thermochemical predictions.
• Explore the potential of MR methods for high-accuracy calculations.

Main Methods:

• Utilized internally contracted multireference coupled cluster (icMRCC) wave functions.
• Employed a full-valence complete active space reference.
• Applied the high-accuracy extrapolated ab initio thermochemistry (HEAT) protocol.
• Tested different icMRCCSD(T) variants.

Main Results:

• icMRCC methods show high accuracy for thermochemical properties, especially atomization energies.
• The intrinsic error of icMRCCSD(T){4}_F is well below the chemical accuracy threshold (4 kJ/mol).
• The methods perform well for molecules with up to 18 correlated electrons.

Conclusions:

• icMRCC methods are suitable alternatives to single-reference wave functions for accurate thermochemistry.
• A multireference framework provides a high-precision tool for computational thermochemical applications.
• These findings support the use of MRCC for challenging electronic structure problems.