Default value

Second card FORMAT(8F10.2), control variables for the regression. This program uses a Newton-Raphson type iteration which is susceptible to convergence problems with poor initial parameter estimates. Therefore, several features are implemented which help control oscillations, prevent divergence, and determine when convergence has been achieved. These features are controlled by the parameters on this card. The default values are the result of considerable experience and are adequate for the majority of situations. However, convergence may be enhanced in some cases with user supplied values. 

BETA cols 11-20 oscillation control parameter default value is set equal to 0.25. To help prevent oscillations (thus slowing convergence) we not only require that the sum of squares, SSQ, decreases... 

PRCG cols 21-30 the maximum allowable change in any of the parameters when LMP = 1, default value is 1000. Limiting the change in the parameters prevents totally unreasonable values from being attained in the first several iterations when poor initial estimates are used. A value of PRCG equal to the magnitude of that anticipated for the parameters is usually appropriate. 

RP cols 31-40 step-back parameter, default value is equal to... 

SSTL cols 41-50 regression convergences control, default value... 

DX(1) cols 1-10 increment for derivative with respect to the temperature, default value is 0,5... 

You cat always use Llic default values. If the ealcii latitin exceeds the iteration limit before it reaches the convergence limit, then most likely there is a convergence failure. Simply increasing this I ini it is ti n I ikely to h elp. Hi e D IIS con vergen ce accelerator rn ay h elp in som e cases. 

Set this threshold Lo a small positive constant (the default value is 10 Hartrcc), Tli is tli resh old is used by HyperCh cm to igu ore all two-cicetron repulsion in tegrals with an absolute value less th an th is value. Tli is option controls the performance of the SCF itera-lious and the accuracy of the wave function and energies since it can decrease the number of ealeulatcd Iwo-elcclrou integrals. 

You can usually use the default values. If the calculation exceeds the iteration limit before it reaches the convergcriec limit, llieti th ere is m osl I ikely con vergeri ce failti re. Sim ply in creasin g tli e limits is un likely to help, I ll e nils COM vergeii ce accelerator, (see SCF Convergence on page 47), wli ich is available for all the SCF... 

The sexlic bending term is a scale factor ST times the t iiadratic bending term. This constant SPcan be set to an arbitrary value by an entry m the Registry or the chem.ini file. The default value for MM+ is SP= 7.0 X 10 . The constant 0.043828 converts the MM-r... 

I h e -M. l+ force field assigns default values for out of plane bending terms around an sp2 center. If a central atom has some out of plane parameters, then the first out of plane parameter involving th at cen tral atom is used if a specific parameter is n ot foiin d. 

The functional form for default torsions is the 1M+ form with th ree torsional constants VI, V2, and V3 for I -fold, 2-fold, and 3-fold contributions. I h e default values for these con stants depend on the particular chemical situatitm associated with theborid... 

I ticse values are from the same referenecs as 2r If a value is not available, a default value of 0.1 keal/mol is used. 

SET CONTROL PARAMETERS (DEFAULT VALUES ARE OVERWRITTEN BY INPUT DATA IF SPECIFIED)... 

The output file eontains more information than the geo file and is documented to be easily understood by the human reader. The first bloek of information (PART 1) is an eeho of the input file with some explanatory notation. The dieleetrie eonstant is arbitrarily set at the default value of 1.5. Default values are autornatieally used in... 

The angles ot, p, and x relate to the orientation of the dipole nionient vectors. The geonieti y of interaction between two bonds is given in Fig. 4-16, where r is the distance between the centers of the bonds. It is noteworthy that only the bond moments need be read in for the calculation because all geometr ic features (angles, etc.) can be calculated from the atomic coordinates. A default value of 1.0 for dielectric constant of the medium would normally be expected for calculating str uctures of isolated molecules in a vacuum, but the actual default value has been increased 1.5 to account for some intramolecular dipole moment interaction. A dielectric constant other than the default value can be entered for calculations in which the presence of solvent molecules is assumed, but it is not a simple matter to know what the effective dipole moment of the solvent molecules actually is in the immediate vicinity of the solute molecule. It is probably wrong to assume that the effective dipole moment is the same as it is in the bulk pure solvent. The molecular dipole moment (File 4-3) is the vector sum of the individual dipole moments within the molecule. 

There are many technical details involved in SCRF calculations, many of which the user can control. Readers of this book are advised to use the default values as much as possible unless they have carefully examined the original literature and tested their modifications. PCM methods are generally more accurate than the Onsager and COSMO methods. 

The step size, Ar, is the maximum allowed atomic displacement used in the generation of trial configurations. The default value of r in HyperChem is 0.05 Angstroms. For most organic molecules, this will result in an acceptance ratio of about 0.5, which means that about 50% of all moves are accepted. 

TheHiickel constant (k) scales the interaction energy between two atomic orbitals (see Extended Huckel Method on page 125). HyperChem uses the default value of 1.75 (see the second part of this book. Theory and Methods). You should use this value for most cases. A suggested range for experimental adjustment of this constant is 1.6-2.0. ... 

In computational chemistry it can be very useful to have a generic model that you can apply to any situation. Even if less accurate, such a computational tool is very useful for comparing results between molecules and certainly lowers the level of pain in using a model from one that almost always fails. The MM+ force field is meant to apply to general organic chemistry more than the other force fields of HyperChem, which really focus on proteins and nucleic acids. HyperChem includes a default scheme such that when MM+ fails to find a force constant (more generally, force field parameter), HyperChem substitutes a default value. This occurs universally with the periodic table so all conceivable molecules will allow computations. Whether or not the results of such a calculation are realistic can only be determined by close examination of the default parameters and the particular molecular situation. ... 

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