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FIELD=(n.nn,m.mm,l.ll)

An external electric field of intensity n.nn volts/Ångstrom in the x-direction, m.mm volts/Ångstrom in the y-direction, and l.ll volts/Ångstrom in the z-direction is to be applied. The potential arising from the field is zero at the origin of Cartesian space, i.e. V=0 at (x=0.0, y=0.0, z=0.0). At any other point, (x,y,z), the effect of FIELD=(n.nn,m.mm,l.ll) is to produce a potential equal to V = (x.n.nn+y.m.mm+z.l.ll) volts.

The effect on a molecule depends on the orientation of the molecule relative to the field.  Thus a hydrogen molecule would be polarized slightly by a field parallel to the axis, but unaffected by a field perpendicular to the axis.  The polarization would be to produce a small positive charge on the hydrogen in the (relatively) negative part of the field, and a small negative charge in the positive part of the field.  Any polarization would, of course, result in the energy being lowered.

Because molecules have no net charge, the effect of an applied field is independent of the location of the molecule.  That is, if the field is defined by FIELD=(1.0,0.0,0.0), the energy of a molecule centered at (+100,0,0) would be the same as that at (-100,0,0) and the same as that at (0,0,0). Put another way, the energy would be unaffected by motion in any direction, including in the direction of the field.

The geometries of small molecules are only slightly affected by fields in the order of 1 volt per Angstrom.  This is because such a molecule is, of its nature, small.  The change in potential from one end of such a molecule to the other would only be a few volts.  Larger systems are affected much more - a system that extends over 20A, say, would experience a potential change of 20 volts, if the field were along that axis.  Macroscopic changes, such as Zener breakdown, are the result of ions being accelerated by the field.  When these ions collide with neutral molecules, the molecules become ionized, e.g. e- + N2 => 2e- +N2+.

On the other hand, an ion will be strongly affected by such a field. For example, F-1 at coordinates 1.00, 0.00, 0.00, would be stabilized by 1.0eV if a field were to be applied by specifying FIELD=(1.0,0.0,0.0). This is a direct consequence of the net charge on the system interacting with the electric field: a charge of +x at a point in space where the potential is y volts would have an energy due to the potential of x.y eV.  This is the origin of the term electron-volt.

A useful exercise is to monitor the behavior of an ion in an electric field, using the DRC:

Let the field be FIELD=(1,0,0), let the ion be F-,  and have a mass of 19 amu.  The data set:

DRC LARGE T-PRIO=0.1 CHARGE=-1 FIELD=(1,0,0) GNORM=0 CYCLES=75
Fluoride ion
In An Electric Field
F 0.0 0 0.0 0 0.0 0

would be suitable here.  After about 63fs, the ion would have moved through 1 Angstrom, and the potential energy would have dropped by 23.06 kcal/mol.  Conservation of energy requires that the kinetic energy equals 23.06 kcal/mol. Using E=(1/2)MV2,  implies that the velocity is:

V = (2*4.184*1010*23.06/19) cm/sec = 3.19*105 cm/sec = 3.19 km/sec.

The 4.184*1010 is the conversion from kcal/mol to ergs per mol. 
All other quantities, such as the acceleration (5.06x1018 cm/sec2 or 5.06x1013 km/sec2), follow simply. 
Used in this way, the DRCis useful for modeling ion implantation.


J. J. P. Stewart
Fujitsu Ltd. 2002