Keyword SCAN

This keyword is used to perform a scan of the potential energy surface along a Cartesian or internal coordinate.
Options:
ADIABATIC / VERTICAL

	ADIABATIC		The start structure for geometry optimization is given by the optimized structure of the previous scan step.
	VERTICAL		The start structure for geometry optimization is given by the initial structure.
	END=$<$Real$>$		End value for scanning coordinate. This specification is mandatory.
	STEP=$<$Integer$>$		Number of scan steps. This specification is mandatory.

Description:
Note that SCAN requires both END and STEP to be specified, even though they have the syntax of options. If a prospective reaction coordinate has been identified, a potential energy surface scan ("linear transit") can be used as a computationally efficient approach for the localization of transition-state starting structures. The options ADIABATIC and VERTICAL of the SCAN keyword are used to select starting structures for a geometry optimization (if requested) along the scanning coordinate. If the keyword OPTIMIZATION (see 4.6.1) is specified, all geometric variables except the scan coordinate are optimized. In the following example, the dihedral angle DX1 is successively reduced from 180.0 to 0.0 degrees in 9 steps. At each step, the variables R1 and R2 are optimized. Their starting values are taken from the previous scan step.

 OPTIMIZATION
 SCAN DX1 END=0.0 STEP=9
 #
 GEOMETRY Z-MATRIX
 C
 N  1  R1
 X  2  RCON  1  ACON
 H  2  R2    3  A1    1  DX1
 #
 VARIABLES
 R1   1.18
 R2   1.01
 DX1 180.0
 #
 CONSTANTS
 RCON  1.0
 ACON 90.0
 A1   90.0

If the keyword OPTIMIZATION had been omitted in the example, the variables R1 and R2 would be kept constant during the potential energy scan. If a Cartesian coordinate is scanned the syntax of the SCAN keyword changes to:

 SCAN H4 END=(0.9,-0.4,-0.3) STEP=10

Here H4 denotes the atom that is moved during the potential energy scan. The END option now defines the final Cartesian coordinates of this atom. These coordinates are kept constant during the optimizations in the individual scan steps. Example on page of the tutorial discusses the input and output of a Cartesian potential energy surface scan in more detail.