Print the topography and Lewis structure. When LEWIS is present, the run is stopped after the structure is printed.
This keyword is useful for checking that a Lewis structure exists. Quite complicated systems can be supplied, such as buckyball (C60), carbon nanotube (buckytube), graphite, phthalocyanines, and diamond.
A Lewis structure consists of single, double and triple bonds, lone pairs, anionic and cationic sites, and positively and negatively charged atoms (ions). The method used in MOPAC is quite powerful, and except for rare cases, gives a reasonable structure. The exceptions are:
In buckytubes, the assumption is made that a graphitic lattice, rather than an extended conjugated poly-acetylene type π bonding structure exists. In some cases, the only unionized structure involves using an extended conjugated poly-acetylene motif.
(To be added, as necessary)
When large organic molecules are being studied, a preliminary calculation to generate the Lewis structure should be run before doing serious work. This will identify all ionized sites in the system. Check that these sites are correct! See also CHARGES
If only the topography is important, then also add keyword 0SCF. This will stop the run after the topography, but before the Lewis structure is printed. This option is useful when using keyword CVB to correct faults in the topology.
Related key-words:
CVB, METAL, VDWM,
CHARGE, CHARGES, SETPI, and MOZYME
See also: Lewis Structures,
MOZYME introduction,
Lewis metal atom charges