MOLPRO

16 THE SCF PROGRAM 103
The atomic density guess works very well with minimal or generally contracted basis sets for
which the first contracted basis functions correspond to the atomic 1s, 2s, 2p ... orbitals, e.g.,
Dunning’s cc-pVnZ sets, the STO-3G, or the 6-31G bases. For such basis sets ATDEN is used
by default. If a segmented basis set with several contractions for each shell is used, ATDEN
should not be specified and H0 is used by default. Since eigenvectors of h are often a very
poor starting guess, it is recommended to generate the starting orbitals using a small basis like
STO-3G (see section 16.4.2 below).
Example:
r=1.85,theta=104
!set geometry parameters
geometry={O;
!z-matrix geometry input
H1,O,r;
H2,O,r,H1,theta}
basis=STO-3G
!first basis set
hf
!scf using STO-3G basis
basis=6-311G
!second basis set
hf
!scf using 6-311G basis set
http://www.molpro.net/info/current/examples/h2o_sto3gstart1.com
The second calculation uses the optimized orbitals of the STO-3G calculation as starting guess.
This is done by default and no START card is necessary. The explicit use of START and SAVE
cards is demonstrated in the example in the next section.
The following input is entirely equivalent to the one in the previous section:
r=1.85,theta=104
!set geometry parameters
geometry={O;
!z-matrix geometry input
H1,O,r;
H2,O,r,H1,theta}
basis=STO-3G
!first basis set
hf
!scf using STO-3G basis
start,atdens
!use atomic density guess
save,2100.2 !save orbitals to record 2100.2
basis=6-311G
!second basis set
hf
!scf using 6-311G basis set
start,2100.2
!start with orbitals from the previous STO-3G calculation.
save,2101.2 !save optimized orbitals to record 2101.2
http://www.molpro.net/info/current/examples/h2o_sto3gstart2.com
16.4.2 Starting with previous orbitals
START,[RECORD=]record.file,[specifications]
reads previously optimized orbitals from record record on file file. Optionally, a specific orbital
set can be specified as described in section 4.11.
The specified dump record may correspond to a different geometry, basis set, and/or symmetry
than used in the present calculation. Using starting orbitals from a different basis set can be
useful if no previous orbitals are available and the ATDENS option cannot be used (see above).
The following example shows how to change the symmetry between scf calculations. Of course,
this example is quite useless, but sometimes it might be easier first to obtain a solution in higher
symmetry and then convert this to lower symmetry for further calculations.