MOLPRO

47 POTENTIAL ENERGY SURFACES (SURF) 351
47 POTENTIAL ENERGY SURFACES (SURF)
SURF,Start1D=label1,options
The SURF program allows for the calculation of the potential energy surface around the equilibrium
structure as required for the calculation of anharmonic frequencies (see the VSCF and
VCI programs). Currently the program is limited to the case of one minimum. The potential
is represented by energy grid points rather than a Taylor expansion. Within the SURF program
the potential energy surface is expanded in terms of normal coordinates, linear combination of
normal coordinates or localized normal coordinates. Consequently, a harmonic frequency calculation
needs to be performed first. The potential will then be represented by a hierarchical
scheme given by
with
V (q 1 ,...,q 3N−6 ) = ∑
i
V i (q i ) +∑V i j (q i ,q j ) + ∑ V i jk (q i ,q j ,q k ) + ... (64)
i< j
i< js
V rs (q r ,q s ) −
∑
r ∈{i, j,k}
V r (q r ) − V (0) (67)
V i jkl (q i ,q j ,q k ,q l ) = ... (68)
where q i denotes the coordinates. This expansion needs to be terminated after an n-body contribution
as controlled by the keyword NDIM. The SURF program is fully parallelized in a sense
that the calculation of different grid points is send to different processors (MPPX scheme). The
START1D keyword is mandatory and defines the label where to jump in the input in order to
do an electronic structure calculation which is terminated by the SURF command. This way the
quality of the potential energy surface is defined.
label1
hf
ccsd
surf,start1D=label1
The SURF program is based on a iterative algorithm, i.e. grid points will be added automatically
to the grid representation of the potential until a convergence threshold will be met. This
guarantees a well-balanced description of the different terms in the expansion of the potential
and simultaneously minimizes the number of ab initio calculations for a representation of the
potential. For further details see:
G. Rauhut, Efficient Calculation of Potential Energy Surfaces for the Generation of Vibrational
Wave Functions, J. Chem. Phys. 121, 9313 (2004).
T. Hrenar, H.-J. Werner, G. Rauhut Accurate Calculation of Anharmonic Vibrational Frequencies
of Medium Sized Molecules Using Local Coupled Cluster Methods, J. Chem. Phys. 126,
134108 (2007).