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16.4.1 Initial orbital guess

An initial orbital guess can be requested as follows:

START,[TYPE=]option

The option keyword can be:

H0
Use eigenvectors of $h$ (core Hamiltonian) as starting guess.
ATDEN
Use natural orbitals of a diagonal density matrix constructed using atomic orbitals and atomic occupation numbers (default).

Note that it is also possible to use orbitals from previous (e.g., smaller basis set) calculations as starting orbitals (see section 16.4.2 below).

Example:

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:

h2o_sto3gstart2.com

Beware, however, that STO-3G is a very poor basis set and usually gives very bad starting vectors. This technique is best used in combination with reasonable small basis sets (e.g., cc-pVDZ or def2-SVP) or minimal basis sets with a proper repesentation of the occupied atomic orbitals (e.g, 6-31G or MINAO/MINAO-PP):

minao_startorb.com



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