# *QUADRATIC RESPONSE

This section gives directives for the calculation of quadratic response
functions [Saue2002a].

## General control statements

### .PRINT

Print level.

*Default:*

## Definition of the quadratic response function

### .B FREQ

Specify frequencies of operator B.

*Example:* 3 different frequencies.

.B FREQ
3
0.001
0.002
0.01

*Default:* Static case.

### .C FREQ

Specify frequencies of operator C (see *.B FREQ*).

### .ALLCMB

Evaluate all nonzero quadratic response functions and thereby
disregarding analysis of overall permutational symmetry.

*Default:* Evaluate only unique, nonzero, response functions.

## Excited state properties

**This page describes unreleased functionality. The keywords may not be
available in your version of DIRAC.**

First order properties of excited states can be computed from the
quadratic response function.

### .EXCPRP

Give the number of “left” and “right” states in each boson symmetry.

*Example*:

Compute the transition moments |\langle i|\hat{A}|i\rangle|,
where i goes from 1 to 5 in each symmetry (four symmetries in this case). The
zeros can be substituted for positive integers to generate elements |\langle i|\hat{A}|j\rangle|.

## Control variational parameters

### .SKIPEE

Exclude all rotations between occupied positive-energy and virtual
positive-energy orbitals.

### .SKIPEP

Exclude all rotations between occupied positive-energy and virtual
negative-energy orbitals.

## Control reduced equations

### .MAXITR

Maximum number of iterations.

*Default:*

### .MAXRED

Maximum dimension of matrix in reduced system.

*Default:*

### .THRESH

Threshold for convergence of reduced system.

*Default:*

## Control integral contributions

The user is encouraged to experiment with these options since they may
have an important effect on run time.

### .CNVINT

Set threshold for convergence before adding SL and SS integrals to
SCF-iterations.

*2 (real) Arguments:*

.CNVINT
CNVXQR(1) CNVXQR(2)

*Default:* Very large numbers.

### .ITRINT

Set the number of iterations before adding SL and SS integrals to
SCF-iterations.

*Default:*

## Control trial vectors

### .XQRNRM

Normalize trial vectors. Using normalized trial vectors will reduce
efficiency of screening.

*Default:* Use un-normalized vectors.

## Advanced/debug flags

### .NOPREC

No preconditioning of initial trial vectors.

*Default:* Preconditioning of trial vectors.

### .RESFAC

New trial vector will be generated only for variational parameter
classes whose residual has a norm that is larger than a fraction
1/RESFAC of the maximum norm.

*Default:*