  We present a state-interaction approach for matrix product state (MPS) wave
functions in a nonorthogonal molecular orbital basis. Our approach allows us to
calculate for example transition and spin-orbit coupling matrix elements
between arbitrary electronic states provided that they share the same
one-electron basis functions and active orbital space, respectively. The key
element is the transformation of the MPS wave functions of different states
from a nonorthogonal to a biorthonormal molecular orbital basis representation
exploiting a sequence of non-unitary transformations following a proposal by
Malmqvist (Int. J. Quantum Chem. 30, 479 (1986)). This is well-known for
traditional wave-function parametrizations but has not yet been exploited for
MPS wave functions.
