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Geometry optimization is a routine job. Below are several recommendations for doing it.

Initial geometry

The optimization cannot be applied to arbitrary geometry. First, the initial geometry should be close to the desired. There should be no considerable overlapping of valance-unbound atoms. The bonds should be of realistic length.

Second, the optimization does not affect the symmetry. For instance, if a molecule was drawn in the screen plane, the optimization cannot displace it from this plane.

Accordingly, one should try to keep to correct scale and conformation when mouse drawing a molecule. Otherwise, the molecule should be edited using either the conformation editor or molecular mechanics optimization. It is recommended not to start drawing from scratch but rather use a convenient model generated previously (in the Kernel box or from file).

Basis selection

Since quantum mechanical calculations are time consuming, it is important to correctly select the basis for a task.

Semiempirical calculation often suffices to correct gross geometry violations. Accordingly, semiempirical optimization is a routine task after mouse drawing a molecule. In this case, a crude gradient can be selected, e.g., 1e-04.

Nonempirical calculation is required to obtain accurate geometry. Typical bases can be found in the Standard panel. The minimum basis provides for sensible bond lengths and valance angles.

If the desired geometry substantially depends on nonvalence interactions, at least the 2d basis is required with an obligatory account for electron correlation. Unfortunately such bases are impractical for large models. In our opinion, if a model is too large to be calculated using MP2 6-311+G(2d,p), the task should be either abandoned or reduced to small model systems to be optimized using MP2 6-311+G(2d,p) and then assembled into a large model, possibly, using molecular mechanics.

Gradient selection

Accurate optimization should be performed until low gradients are reached, at least, 1e-06 Hartree/Bohr. The optimization of van der Waals molecules requires gradients of 1e-07 - 1e-08. Unfortunately, numerical accuracy problems arise in this case. Such problems are typically indicated by the oscillations of energy or geometry and procedural convergence failure. In this case, one should first try to switch the Calc mode from Fast to Accurate. This decelerates the optimization but can improve the geometry in many cases. Otherwise, one should consult the GAMESS documentation and try different optimization methods, HF calculation options, and coordinate systems.


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