A tool for monitoring the behavior of SCF calculations is useful when they take
too long, or even fail altogether. When keyword PL is present, the energy of the system and the
rate of change of the electronic structure can be monitored iteration to
iteration. This keyword
is useful particularly when trying out various combinations of convergers such
as PULAY, KING, SHIFT, and DAMP
to find the method that works best.

The rate of change in the electronic
structure is given by the quantity PLS in the output. For example, in the
line:

ITERATION 7 PLS= 0.168E-01 0.395E-06 ENERGY -34.585270 DELTAE -2.2653063

the alpha wavefunction changed by 0.0168 between iteration 6 and iteration 7. At the same time, the beta wavefunction changed by 0.000000395.
The change in energy over these iterations is -2.265 kcal/mol.

If the
calculation uses a restricted Hartree-Fock method, then the line would look like
this:

ITERATION 7 PLS= 0.900E-02 0.000E+00 ENERGY 46.712559 DELTAE -0.3203785

Now the change in total wavefunction between iteration 6 and 7 would be
0.009. The second number, here 0.000E+00, is not used and should be
ignored.

The precise meaning of PLS depends on whether the calculation uses
MOZYME or the conventional, default, method.

Definition of PLS for conventional SCF calculations

In conventional methods, PLS is the largest change in any density matrix element on two successive iterations
in the SCF calculation. At self-consistency, this change drops to zero.
For UHF calculations, both the alpha and beta density matrices are used,
therefore two numbers are printed. The total density matrix is used in RHF
calculations, and consequently the second number is not used.

Definition of PLS for MOZYME calculations

MOZYME calculations do not use a normal density matrix so the definition used
above cannot be used. Instead, the largest Fock matrix element connecting
an occupied LMO with any virtual LMO is used. This can be written as:

PLS = |<y^{occ}|F|y^{vir}>|

Although this definition is fundamentally different from that in conventional
work, in that data from only one iteration is used and the units are now
electron volts and not electrons, the meaning is the same - at self-consistency
all Fock matrix elements connecting occupied and virtual LMOs are zero.
Therefore, from a user's perspective, the significance of PLS in both
conventional and MOZYME calculations is the same: it is a measure of how far the
system is from self-consistency.