The influence of the electronic structure method on intersystem crossing dynamics. The case of thioformaldehyde
2019-05-17T13:35:02Z (GMT) by
The ability of different electronic structure methods to describe correctly intersystem crossing dynamics is evaluated, using thioformaldehyde as a test case. Mischievously, all methods considered—ranging from the multi-reference methods MRCISD, MS-CASPT2, or SA-CASSCF, to the single-reference methods ADC(2), CC2, and TDDFT in different flavours— provide the same state ordering and energies of the low-lying singlet and triplet electronic excited states within an acceptable error of 0.2–0.3 eV. However, the outcome of the non-adiabatic simulations after excitation to the lowest S1 (1nπ∗) state are dramatically different. While MS-CASPT2, ADC(2), BP86, and PBE do not transfer population to the triplet states within 500 fs—in consonance with experimental evidence—SA-CASSCF, B3LYP, and BHHLYP predict intersystem crossing yields between 3% and 21% within the same time. The different excited state dynamics can be rationalized by inspecting potential energy profiles along the C–S bond stretch mode and single-triplet energy gaps. It is found that already at a C–S bond length of 1.9Å, all the single-reference methods struggle to describe the correct asymptotic behavior of the potentials. Moreover, some methods, including SACASSCF, obtain incorrect 1nπ∗ − 3ππ∗ energy gaps, leading to compensation of errors (ADC(2), BP86, PBE), or wrong dynamics (SA-CASSCF, B3LYP, BHHLYP). Only the accurate MRCISD and MS-CASPT2 methods are able to describe the C–S bond correctly and thus able to deliver the correct potential energy surfaces and dynamics for the right reason. A correlation with the amount of Hartree-Fock exchange in the density functional and the easiness to access the 3ππ∗ state from the 1nπ∗ is able to explain the different behavior observed for GGA and hybrid functionals. It is thus illustrated that even in the case of a simple molecule, like CH2S, the sole assessment of vertical excitation energies as reliability predictors for non-adiabatic is inadequate. The reason is that ISC does not occur at the FC geometry, but rather at distorted geometries where the singlet-triplet gaps become small. Hence, a characterization of the potential energy surfaces beyond the Franck-Condon region is mandatory.