.. _CUTOFF: ``CUTOFF`` ========== | ```MOZYME`` `__ calculations run faster when some NDDO interactions are ignored.  By default, all NDDO interactions are included between atoms separated by 9.9 Ångstroms or less, and all polarization functions between atoms separated by 10 Ångstroms or less.  For rough work, such as optimizing a structure that is far from a minimum, if ``CUTOFF=n.nn`` is used, then only point charge interactions beyond *n.nn* Ångstroms are used.  A suitable value for *n.nn* is 6.0.  This will give a large increase in speed. | Before using\ ``CUTOFF`` to increase speed, first try using ``CUTOFP.`` 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\ :sub:`202`\ H\ :sub:`315`\ N\ :sub:`55`\ O\ :sub:`64`\ S\ :sub:`6`, see Table I.  For a larger system, hexokinase, 2E2O, a medium-sized protein of 5717 atoms and 299 residues, C\ :sub:`1448`\ H\ :sub:`3073`\ N\ :sub:`375`\ O\ :sub:`810`\ S\ :sub:`11`, results for a 1SCF calculation are shown in Table II.  Hexokinase contains 375 molecules of water. .. raw:: html
Table I: Crambin .. raw:: html
    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``. ``CUTOF1=n.nn`` --------------- 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 ``CUTOF1``, electrostatic interactions are represented by a point-charge and three polarization functions. Default: For systems of less than 30 atoms, ``CUTOF1`` is 1000 Ångstroms; for systems of 30 or more atoms, ``CUTOF1`` is 10 Ångstroms. ``CUTOF2=n.nn`` --------------- 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 <*ss*\ \|\ *ss*>, <*ss*\ \|\ *sx*>, <*ss*\ \|\ *sy*>, <*ss*\ \|\ *sz*>, <*sx*\ \|\ *ss*>, <*sy*\ \|\ *ss*>, and <*sz*\ \|\ *ss*>.  At still greater distances, beyond ``CUTOF1``, only the <*ss*\ \|\ *ss*> term is used. Default: for systems of less than 30 atoms, ``CUTOF2`` is 1000 Ångstroms; for systems of 30 or more atoms, ``CUTOF2`` is 9.9 Ångstroms.