Spin-crossover in Molecules and Solids with a Low-cost, Well-behaved meta-GGA Density Functional*

Recently the SCAN meta-GGA exchange-correlation functional has been revised to eliminate significant numerical instabilities while maintaining constraint satisfaction. The resulting r²SCAN functional [1] is shown to give substantially better spin-crossover (SCO) electronic energies (high spin minus low spin) on a benchmark data set than the original SCAN as well as on some Fe complexes [2]. The deorbitalized counterpart r²SCAN-L [3] is not as good as r²SCAN but almost as good as the original SCAN. A crucial advantage, however, is that r²SCAN-L is much faster than r²SCAN in periodically bounded systems. A combination strategy for balanced treatment of molecular and periodic system spin-crossover therefore is to optimize geometry with r²SCAN-L, then calculate SCO energies with r²SCAN. We present examples.
[1] J. Phys. Chem. Lett. 11, 8208 (2020).
[2] J. Phys. Chem. A (submitted); arXiv 2009.07808
[3] Phys. Rev. B 102, 121109(R) (2020).