17.3.1 Initial orbital guess

An initial orbital guess can be requested as follows:

START,[TYPE=]option

The option keyword can be:

H0

Use eigenvectors of $h$ as starting guess.

ATDEN

Use natural orbitals of a diagonal density matrix constructed using atomic occupation numbers.

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 $1s$, $2s$, $2p$ $\ldots$ 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 13.3.2 below).

Example:

Input: h2o_sto3gstart1.com

Output: h2o_sto3gstart1.out

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:

Input: h2o_sto3gstart2.com

Output: h2o_sto3gstart2.out