posted on 2016-02-19, 00:50authored byNils Wittenbrink, Hameth Ndome, Wolfgang Eisfeld
The theoretical treatment of state–state
interactions and
the development of coupled multidimensional potential energy surfaces
(PESs) is of fundamental importance for the theoretical investigation
of nonadiabatic processes. Usually, only derivative or vibronic coupling
is considered, but the presence of heavy atoms in a system can render
spin–orbit (SO) coupling important as well. In the present
study, we apply a new method recently developed by us (J.
Chem. Phys.2012, 136, 034103,
and J. Chem. Phys.2012, 137, 064101) to generate SO coupled diabatic PESs along the C–I
dissociation coordinate for methyl iodide (CH3I). This
is the first and mandatory step toward the development of fully coupled
full-dimensional PESs to describe the multistate photodynamics of
this benchmark system. The method we use here is based on the diabatic
asymptotic representation of the molecular fine structure states and
an effective relativistic coupling operator. It therefore is called
effective relativistic coupling by asymptotic representation (ERCAR).
This approach allows the efficient and accurate generation of fully
coupled PESs including derivative and SO coupling based on high-level ab initio calculations. In this study we develop a specific
ERCAR model for CH3I that so far accounts only for the
C–I bond cleavage. Details of the diabatization and the accuracy
of the results are investigated in comparison to reference ab initio calculations and experiments. The energies of
the adiabatic fine structure states are reproduced in excellent agreement
with ab initio SO–CI data. The model is also
compared to available literature data, and its performance is evaluated
critically. This shows that the new method is very promising for the
construction of fully coupled full-dimensional PESs for CH3I to be used in future quantum dynamics studies.