Contents: Table of Contents   Distributed Polarizabilities  Chapter 6  External fields and lattices

Origin and Gauge Dependence

It is possible to change the origin of the molecular coordinates after the geometry has been read in. There are three subvariations,

ORIGIN COM

moves the origin to the centre-of-mass

ORIGIN COC

moves the origin to the centre of the nuclear charge (not recommended unless you want to confuse yourself when evaluating properties) and

bf ORIGIN X Y Z

moves the origin to the cartesian position (X,Y,Z), where the values are in atomic units.

Note that the ORIGIN command translates the whole molecule to a new origin, but it does so only once, so that if you set the origin at the centre of mass, and then do a geometry optimisation, there is no guarantee that the origin is still at the centre of mass. Note that some molecular properties depend upon the choice of origin e.g. only the first non-zero multipole moment is independent of origin - higher moments e.g. the quadrupole moment in a dipolar molecule, depend upon the origin.

Normally the origin with respect to which properties are evaluated coincides with the centre of the molecular coordinate system. However it is possible to change this with the command,

GAUGE X Y Z

which sets a new origin for properties (and a new gauge for the magnetic field).

Note that whereas ORIGIN translates the molecule, GAUGE leaves the molecule fixed in space. Changing the gauge will primarily effect magnetic properties.

Note that the two commands are not independent eg the following data sets

TITLE
Calculation of Properties
SYMMETRY
CNV 2
END
VARIABLES
OH 0.956 A
HOH 52.25 D
END
BASIS DZP
ATOMS
OXYGEN 8.0 0.0 0.0 0.0
HYDROGEN 1.0 POL OH HOH 0.0
END

GAUGE 0.00000000      0.00000000      0.12377989
MAGNETISAB
START
FINISH

and that with the GAUGE line replaced by

ORIGIN COM

have the same effect One should therefore be careful not to include both commands unless you are sure what is going on!