Keyword MOEXCHANGE

This keyword alters the molecular orbital ordering in the start or restart density.
Options:

	$<$Integer1$>$		Number of $\alpha$ molecular orbital exchanges.
	$<$Integer2$>$		Number of $\beta$ molecular orbital exchanges.

Description:
The indices of the molecular orbitals to be exchanged are read in pairs of integers in the keyword body of MOEXCHANGE, one line for each pair. First all $\alpha$ exchanges (or MO exchanges in case of RKS and ROKS) are read, and then, in the case of an unrestricted calculation (see UKS in 4.5.1) $\beta$ exchanges are read. A maximum of 10 MO exchanges in each spin manifold is permitted. The following N$_2$ input shows how the start density, which here is the tight-binding density, can be altered with MOEXCHANGE.

 Multiplicity 3
 SCFType ROKS Max=100
 VxcType Basis BLYP
 Mixing 0.4
 Guess TB
 MOExchange 1
 7 9
 PRINT MOs = 6-9
 FIXMOS SCF=1 Fixed
 Geometry Z-Matrix
 N
 N  N  R
 #
 Variables
 R  1.0975
 Basis (STO-3G)
 End

In this example orbitals 7 and 9 are exchanged. In the output (see below), the orbitals 6 to 9 of the tight-binding start density are printed. This output is triggered by the PRINT keyword with the option "MOs = 6-9" (see 4.12.2). The exchange of orbital 7 and 9 results in an unoccupied orbital (9) with lower energy than that of the two highest occupied orbitals which are degenerate. Here is the output of the tight-binding orbitals before the SCF iteration:

 TIGHT-BINDING MO COEFFICIENTS OF CYCLE 1


                               6          7          8          9
                            -.3043      .2841      .2841     -.1589

                            2.0000     1.0000     1.0000      .0000

    1    1   N    1s         .0000      .0000      .0000      .0413
    2    1   N    2s         .0000      .0000      .0000     -.3246
    3    1   N    2py        .5622     -.8336     -.0006      .0000
    4    1   N    2pz        .0000      .0000      .0000      .6345
    5    1   N    2px        .2728     -.0006      .8336      .0000

    6    2   N    1s         .0000      .0000      .0000      .0413
    7    2   N    2s         .0000      .0000      .0000     -.3246
    8    2   N    2py        .5622      .8336      .0006      .0000
    9    2   N    2pz        .0000      .0000      .0000     -.6345
   10    2   N    2px        .2728      .0006     -.8336      .0000

Clearly, this is an excited state configuration which will not be preserved during the SCF iterations. With the keyword FIXMOS (see 4.4.5) this configuration can be fixed during the SCF iteration. With the options "SCF=1" and "Fixed" the configuration of the first SCF cycle (here the above displayed tight-binding MOs) is fixed and retained through the SCF. The converged molecular orbitals are also printed:

 MO COEFFICIENTS OF CYCLE 8


                               6          7          8          9
                            -.3091      .0647      .0647     -.3023

                            2.0000     1.0000     1.0000      .0000

    1    1   N    1s         .0000      .0000      .0000      .0741
    2    1   N    2s         .0000      .0000      .0000     -.4167
    3    1   N    2py        .6205      .8129      .1848      .0000
    4    1   N    2pz        .0000      .0000      .0000      .5995
    5    1   N    2px       -.0736     -.1848      .8129      .0000

    6    2   N    1s         .0000      .0000      .0000      .0741
    7    2   N    2s         .0000      .0000      .0000     -.4167
    8    2   N    2py        .6205     -.8129     -.1848      .0000
    9    2   N    2pz        .0000      .0000      .0000     -.5995
   10    2   N    2px       -.0736      .1848     -.8129      .0000

Note the orbital energy of the lowest unoccupied MO.