Parallel multireference configuration interaction calculations on mini-beta-carotenes and beta-carotene

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• The multireference configuration interaction (MRCI) method is crucial for accurate electronic structure calculations.
• Parallelization of computational codes is essential for tackling large-scale scientific problems.
• Density functional theory (DFT) combined with MRCI (DFT/MRCI) offers a computationally feasible approach for excited state studies.

Purpose of the Study:

• To present a parallelized direct selecting MRCI code for enhanced computational efficiency.
• To investigate the performance of the parallelized code on different cluster architectures.
• To apply the DFT/MRCI method to study the electronic states of carotenoids.

Main Methods:

• Parallelization of a direct selecting MRCI code.
• Implementation of both ab initio and semiempirical DFT/MRCI modes.
• Case studies on carotenoids and porphyrins to assess parallelization efficiency.
• Investigation of low-lying singlet and triplet states of mini-n-beta-carotenes and beta-carotene using DFT/MRCI.

Main Results:

• Parallelization efficiency is highly dependent on cluster architecture, with near-linear speed-up on older systems but memory bandwidth limitations on modern multi-core processors.
• Recommendations are provided for optimal MRCI process distribution on modern CPUs to avoid memory saturation.
• The DFT/MRCI method accurately reproduces energy gaps between excited states of carotenoids, though absolute excitation energies are slightly underestimated for longer chains.
• Calculated triplet-triplet absorption energies for beta-carotene show excellent agreement with experimental values.

Conclusions:

• The parallelized MRCI code offers a valuable tool for electronic structure calculations, with performance optimized by considering hardware architecture.
• The DFT/MRCI method is a reliable approach for studying the excited state properties of conjugated systems like carotenoids.
• Computational strategies should account for memory bandwidth limitations on modern processors for memory-intensive tasks.