If `CUTOFF=6` is used, then `
GNORM=20` is recommended.
Once the geometry is optimized using `GNORM=20`,
the `CUTOFF` should be removed, and `GNORM` reduced to,
e.g., `GNORM=10`.

The effect of `CUTOFF` on times and accuracy of prediction of geometry can be
investigated by examining the statistics for the optimization of the positions of hydrogen atoms in
Crambin, a small 642 atom, 46 residue protein with formula C_{202}H_{315}N_{55}O_{64}S_{6},
see Table I. For a larger system, hexokinase, 2E2O, a medium-sized protein of 5717 atoms and 299
residues, C_{1448}H_{3073}N_{375}O_{810}S_{11},
results for a 1SCF calculation are shown in Table II. Hexokinase contains 375 molecules of water.

Table I: Crambin | Table II: Hexokinase | ||||||||||||

Cutoff | Time (s) | Ratio | Distortion (Å) | Distortion (Å) | Energy (Kcal/mol) |
Relative energy |
Cutoff | Time (s) | Ratio | Energy (Kcal/mol) |
Relative energy |
||

Default | 1328 | 1.00 | 49.5 | 0.0 | -3342.2 | 0.0 | Default | 774 | -45095.0 | 0.0 | |||

7.0 | 746 | 0.56 | 50.0 | 1.7 | -3342.5 | -0.3 | 12.0 | 796 | -45095.0 | 0.0 | |||

6.0 | 889 | 0.76 | 49.6 | 1.3 | -3342.1 | +0.1 | 11.0 | 746 | -45095.1 | -0.1 | |||

5.0 | 599 | 0.45 | 46.3 | 8.8 | -3334.5 | +7.7 | 10.0 | 685 | -45095.1 | -0.1 | |||

4.0 | 348 | 0.26 | 51.7 | 5.0 | -3315.8 | +26.4 | 9.0 | 599 | -45095.2 | -0.2 | |||

3.0 | 119 | 0.09 | 34.9 | 26.7 | -2952.1 | +390.1 | 8.0 | 546 | -45095.6 | -0.4 | |||

no-opt | - | 0.0 | 49.5 | -2963.6 | N/A | 7.0 | 490 | -45096.7 | -1.7 | ||||

6.0 | 343 | -45093.3 | +1.7 | ||||||||||

5.0 | 309 | -50783.8 | -5688.8 | ||||||||||

4.0 | 252 | -44949.3 | +145.7 |

For the rare occasions when finer control is needed, use `CUTOF1` and `CUTOF2`.

In `MOZYME` calculations, the cutoff distance for polarization
functions is set by `CUTOF1= n.nn`. Beyond that
distance, electrostatic interactions are considered only as point
charges. At distances less than that given by

In `MOZYME` calculations, the cutoff distance for two-electron
two center and one-electron two center integrals is set by
`CUTOF2= n.nn`. Below that distance, the interaction between two
atoms is represented by the exact NDDO approximation. Above that
distance, one-electron integrals that depend on the overlap are
ignored, and two-electron integrals are simplified. Instead of using
all 100 two-electron integrals between two heavy atoms, only seven are
used. These represent the terms <