Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 133
proaches but cases where current exchange-correlation functionals dramatically
fail are known to exist [94, 95].
The calculated dipole transition moments are very sensitive to the quality
of the calculation, namely the basis sets and the functional used [96]. Obviously
they rely on the hypothesis that DFT excited states are well defined, an
assumption which is not always true as explained above. Some functionals
have been found to be more sensitive to the basis set quality than others.
2.5
Potential Energy Surfaces
In principle photochemical mechanisms are described by reaction paths on
ground and excited states PES which can be determined according to the
procedures developed for chemical reactivity. The new problem in photo-induced
mechanisms is the complicated landscape of the PES characterized by
the presence of a variety of critical geometries such as minima, transition
states, high-order saddle points, avoided crossings, conical intersections resulting
from non-adiabatic interactions between N-Dimensional PES. For a
given atomic basis set the computational method used to solve the electronic
problem has to be flexible enough to characterize correctly different regions
of the molecular PES at the same level of accuracy. An inadequate wave
function would result in a biased description of the different regions and
such computed PES would not reproduce the exact BO potentials.
One of the most significant advances made in applied quantum chemistry
in the past 20 years is the development of computationally workable
schemes based on the analytical energy derivatives able to determine stationary
points, transition states, high-order saddle points and conical intersections
on multidimensional PES [8]. The determination of equilibrium geometries,
transition states and reaction paths on ground state potentials has
become almost a routine at many levels of calculation (SCF, MP2, DFT, MC-
SCF, CCSD, CI) for molecular systems of chemical interest [97, 98]. The
availability of reliable and efficient analytic energy gradient procedures (first
and second derivative) for the search of various critical points on several interacting
complex surfaces at high correlated level (CASSCF, CCSD(T) and
its extension EOM-CCSD, MR-CI) will have a significant impact in the theoretical
study of transition metal photo-reactivity. Indeed, although reaction
paths are uniquely defined in any coordinate system they cannot be determined
unambiguously without the knowledge of reference points on the PES
from which the analytical energy derivation procedure may start. Unfortunately
the derivative formulation for highly correlated wave functions is very
complex and if analytical first derivatives are available for the standard
methods second derivatives calculations are even more complicated.
Density functional methods are competitive with the above traditional
wave functions methods for numerous applications among them the compu-