General keywords

$drvopt

is the keyword for non-default options of gradient and second derivative calculations. Possibilities in case of the module aoforce are:

frequency analysis only

analysis only

to read a complete Hessian from the input file $hessian and perform only the frequency analysis

analysis [only] intcoord [print printlevel]

to perform an analysis of normal modes in terms of internal coordinates. Details about this option and the effect of the printlevel (default is 0) are given in Section 11. The effect of the keyword only is the same as described above.

$maxcor 50

fixes the RAM memory to be used by the run (here 50MB), about 70% of available memory should be fine, because $maxcor specifies only the memory used to store derivatives of density and Fock matrices as well as the CPHF-RHS. Default is 200MB.

$forceconv 7

sets the convergence criterion for the CPHF-equations to a residual norm of 1.0d-7. Normally the default value of 1.0d-5 already provides an accuracy of vibrational frequencies of 0.01cm^-1 with respect to the values obtained for the convergence limit.

$forceiterlimit 10

fixes the maximum number of Davidson iterations for the solution of the CPHF-equations to a value of ten. Normal calculations should not need more than eight iterations, but as a precaution the default value is 25.

$nosalc

forces the program in case of molecules with C₁ symmetry not to use 3N - 6(5) symmetry adapted but all 3N cartesian nuclear displacement vectors. This option may lead to a moderate speed-up for molecules notedly larger than 1000 basis functions and 100 atoms.

$noproj

forces the program not to project out translations and rotations when forming a basis of symmetry adapted molecular displacements. This option may be needed if a Hessian is required, that contains translation- and rotation-contributions, e.g. for coupling the system with low cost methods. Output of the unprojected hessian is done on $nprhessian; format is the same as for conventional $hessian. Output of the corresponding eigenvalues and eigenvectors is done analogously on $nprvibrational spectrum and $nprvibrational normal modes.

$nomw

causes the program to diagonalize a not mass weighted hessian. Output is on $nprhessian, $nprvibrational spectrum and $nprvibrational normal modes, because projection of rotations is not possible in this case.

$isosub

This keyword allows to trace back the effects of isotopic substitution on vibrational frequencies. The atom(s) for which isotopic substitution is to be investigated are specified in subsequent lines of the form (atom index) (mass in special isotope), e.g.

$isosub 
 3 2.001 
 5 13

The interpolation then takes place between the mass(es) specified in $atoms (or the default mass(es), if none specified) and the mass(es) in $isosub. Take care of symmetry equivalent atoms, otherwise symmetry analysis will fail. This feature can not be used in a lowest eigenvalue search (keyword $les).

$isopts 6

Sets the number of points for interpolation between the two isotopes compared by the $isosub option to six. Default value is 21.

Keywords for the treatment of only selected nuclear displacement vectors:

$ironly

CPHF-iteration is done only for distortions, that are IR active.

$ramanonly

CPHF-iteration is done only for distortions, that are Raman active.

$les

This causes a lowest Hessian eigenvalue search to be performed instead of a complete force constant calculation. The lowest eigenvalue search consists of the calculation of a guess-Hessian and macro-iterations to find the solution vector(s) for the lowest eigenvalue(s). In each macro-iteration the CPHF-equations are solved for the present search vector(s). $les all 1 means that one lowest eigenvalue for each irrep will be determined, other numbers of lowest eigenvalues per irrep are admissible too.

Different numbers of lowest eigenvalues for different irreps are requested by e.g.

$les
a1 3
a2 all
b2 1

The convergence criterion of the Davidson iterations for the solution of the CPHF-equations as well as the maximal residual norm for the lowest Hessian eigenvalue in the macro-iteration are specified by $forceconv as explained above.

The maximum number of macro-iterations is specified by $lesiterlimit x

with the default x=25. The maximum number of iterations for each solution of the CPHF-equations is again determined by $forceiterlimit as shown above.

The convergence of the macro-iterations is strongly influenced by the size of the starting search-subspace. Generally all guess-Hessian eigenvectors corresponding to imaginary frequencies and at least two real ones are used as starting search-subspace. However it proved to be necessary to use even more vectors in the case of guess-Hessians with very large conditioning numbers.
$hesscond 8.0d-5
means that all eigenvalues with the quotient (eigenvalue)/(max. eigenvalue) lower than 0.00008 are added to the starting search-subspace. Default is 1.0d-4.

Force constant calculations on the DFT level prove to be numerically reliable only with large integration grids or if one includes the effects of quadrature weights. This is done by default--to prevent this, insert
no weight derivatives
in $dft.