The `SURF` program allows for the calculation of the potential energy surface
around a reference structure as required for the calculation of
anharmonic frequencies (see the `VSCF` and `VCI` programs).
The reference structure is supposed to be a (local) minimum or a transition state of a double-minimum potential.
The potential is represented by energy grid points rather than
an analytical representation.
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 multi-mode expansion, i.e. a hierarchical scheme given by

where denotes the coordinates. This expansion needs to be terminated after an -body contribution as controlled by the keyword

label1 hf ccsd surf,start1D=label1

The `SURF` program is based on an 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).

- 52.1 Options
- 52.2 Multi-level calculations
- 52.3 Special options for Intensities
- 52.4 Error correction schemes
- 52.5 Restart capabilities
- 52.6 Linear combinations of normal coordinates
- 52.7 Scaling of individual coordinates
- 52.8 Deleting individual surfaces
- 52.9 Modeling of high-order -body terms
- 52.10 Quality Check
- 52.11 Grid Computing
- 52.12 Recommendations
- 52.13 Standard Problems