Example: Electric Field, Point Charge(s): N2 and PtCO¶

Two illustrations of applying the very useful BASIS keyword and of application of an Electric Field.

For N2, three calculations are provided: 1) a normal N2 run as a reference with the BASIS keyword, 2) with a homogeneous electric field, 3) with a point charge.

In this example, no Create run is needed in the input file, because the first molecular calculation uses the BASIS keyword. If the $ADFBIN/adf script finds this keyword, it will first generate a new input file which will then be executed. The new input file will contain the required Create run for the N atom in this case. The proper xc functional and relativistic options will automatically be selected by the BASIS keyword. This includes Dirac calculations in case of relativistic runs. The output files is identical to what would have appeared if one would provide the Create runs explicitly in the input file. It also copies the atomic input, so that everything can be checked. $ADFBIN/adf -n1 << eor
title N2  reference for comparison with E-Field runs

atoms
N  0 0 -.55
N  0 0 +.55
end

Basis
Type DZP
Core Small
End

end input
eor

rm TAPE21 logfile

$ADFBIN/adf << eor scf conv 1e-8 end title N2 in a homogeneous electric field atoms N 0 0 -.55 N 0 0 +.55 end fragments N t21.N end EField 0 0 0.01 end input eor rm TAPE21 logfile$ADFBIN/adf << eor
title  N2 polarized by a point charge on the axis

EField
0 0 3.0   1.0
end

atoms
N 0 0 -.55
N 0 0 .55
end

Fragments
N  t21.N
end

endinput
eor


In the second n2 run the homogeneous field is supplied with the key efield, used as simple key: one record, data on the same line as the keyword. The field strength is specified in atomic units.

Homogeneous electric fields can be used to study the polarizability: for sufficiently small fields the dipole moment should respond linearly.

For point charges, the third calculation, the block form of the key efield must be used. The program first tries to find data on the same line as the keyword (defining a homogeneous field). If this is absent, a data block is expected with point-charge specifications: x, y, z and q.

The coordinates are in the same units as in the atoms block (angstrom by default) (but always Cartesian). Q is the charge in elementary units (+1 for a proton).

Point charges can be used for instance to simulate crystal fields (Madelung potential).

Note: the symmetry will be determined automatically by the program as C(lin), rather than D(lin), in the two runs that involve an electric field: the fields break the symmetry.

For PtCO, a fairly large electric field is applied in combination with a tight SCF convergence criterion.

The BASIS keyword in this example illustrates how different choices can be made for different atoms (in this case a frozen core for Pt).

Basis
Type DZ
Core None
Pt Pt.4d
END