Locating Minimum Energy Crossings of Different Spin States Using the Fragment Molecular Orbital Method

Spin-dependent processes involving
nonadiabatic transitions between
electronic states with different spin multiplicities play important
roles in the chemistry of complex systems. The rates of these processes
can be predicted based on the molecular properties at the minimum
energy crossing point (MECP) between electronic states. We present
the development of the MECP search technique within the fragment molecular
orbital (FMO) method applicable to large complex systems. The accuracy
and scalability of the new method is demonstrated on several models
of the metal–sulfur protein rubredoxin. The effect of the model
size on the MECP geometry and relative energy is discussed. The fragment
energy decomposition and spin density delocalization analyses reveal
how different protein residues and solvent molecules contribute to
stabilization of the spin states. The developed FMO-MECP method can
help to clarify the role of nonadiabatic spin-dependent processes
in complex systems and can be used for designing mutations aimed at
controlling these processes in metalloproteins, including spin-dependent
catalysis and electron transfer.