In some instances, the default Lewis structure used in a MOZYME calculation is not correct.  The correct structure can be selected by explicitly supplying one or more π bonds.  To do this, add keyword SETPI or SETPI.TXT, both mean the same thing.  After the data set, add  the atom-pairs, see atom numbering, that define one or more π bonds.  For example, a hexagon of carbon atoms with four hydrogen atoms and two oxygen atoms in the 1 and 4 positions, i.e. C6H4O2, can either be the neutral molecule para-benzoquinone or the di-anion of hydroquinone.  The default Lewis structure is the neutral system.  If the di-anion is wanted, then the three aromatic π bonds need to be explicitly defined, thus:
Mozyme setpi charge=-2 
  Di-anion of hydroquinone

  C     0.00000000  0    0.0000000  0    0.0000000  0                      
  C     1.42891759  1    0.0000000  0    0.0000000  0     1     0     0    
  C     1.39123760  1  121.7152363  1    0.0000000  0     2     1     0    
  C     1.42884933  1  121.6862338  1    0.0522686  1     3     2     1    
  C     1.42731168  1  116.6123504  1   -0.0326433  1     4     3     2    
  C     1.39298952  1  121.7002715  1    0.0291866  1     5     4     3    
  O     1.28854675  1  121.6283321  1  179.9905509  1     1     2     3    
  O     1.28863750  1  121.5839611  1  179.9914815  1     4     3     2    
  H     1.07825816  1  117.6731944  1 -179.9642741  1     2     1     3    
  H     1.07822008  1  120.6215632  1 -179.9762574  1     3     2     1    
  H     1.07805743  1  117.7610380  1 -179.9848142  1     5     4     3    
  H     1.07809519  1  120.5409971  1  179.9757231  1     6     5     4   
2 3
1 6
4 5

When large biomolecules are being run, defining atoms by their atom number is sometimes difficult.  In a PDB atom label, the number may or may not be the same as the atom number.  Two simpler methods for defining atoms are provided: the PDB label and the JSmol label.  If the PDB or JSmol label is used, then enclose the label in quotation marks. Wildcards, where the chain letter is replaced by an asterisk, or a residue is replaced by three asterisks, are allowed. The first occurrence that matches will be used. Examples of valid labels are:

Examples of the use of PDB and JSmol
definitions of atoms used in π-bonds
"[8OG]1157:A.C6" "[8OG]1157:A.N1"
"CB PHE B 74" "CG PHE B 74"
"[8OG]1157:A.C6" "N1 8OG B1157"
The same examples, with wildcards
"[***]1157:A.C6" "[***]1157:*.N1"
"CB PHE * 74" "CG PHE * 74"
"[8OG]1157:*.C6" "N1 *** B1157"
Quotation marks are part of the definition

An alternative to putting the π-bonds at the end of the dataset is to define them in a file and specify the file using SETPI=<text>.txt, for example, SETPI=pibonds.txt. In this case, the file pibonds.txt would contain the same data as would have been at the end of the data set.  This format must be used if GEO_DAT is used. There is no need to supply all the π bonds, only those necessary to resolve any ambiguities.  Other examples of the need for SETPI include:

Distinguishing the anion of p-hydroxy N-methyl pyridine from its quinone type tautomer.
Ensuring that  meso-tetra(para-N-methylpyridinato)-porphyrin has the correct charge.

Worked example of Guanine anion

Guanine (see top figure) anion can be made by removing a proton from either C6 or N1. If the only guide to the electronic structure is the topology, then both of the lower figures are valid.  If a double bond is made between C6 and O6 (bottom left figure) then N3 would have only two valencies used, instead of the three valencies expected.  This is obviously wrong but it follows logically from the presence of the C=O double-bond.  In MOPAC, by default, a nitrogen atom with a valence other than 3 would be assigned a positive charge. This is the wrong charge. To prevent this from happening, SETPI could be used to define a double bond between C6 and N1.  If that is done (bottom right figure) then all the faults are corrected.  C6 now automatically forms a single bond with O6, which then automatically gets a negative charge, the ring becomes aromatic, and the faulty divalent nitrogen becomes trivalent.



See also CVB to explicitly make or break bonds, and atom labels to explicitly assign charges.