# *QUADRATIC RESPONSE¶

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

## Definition of the quadratic response function¶

### .DIPLEN¶

Specification of dipole operators for A, B, and C
(see *One-electron operators* for details).

### .A OPERATOR¶

Specification of the A operator
(see *One-electron operators* for details).

### .B OPERATOR¶

Specification of the B operator
(see *One-electron operators* for details).

### .C OPERATOR¶

Specification of the C operator
(see *One-electron operators* for details).

### .B FREQ¶

Specify frequencies of operator B.

*Example:* 3 different frequencies.

```
.B FREQ
3
0.001
0.002
0.01
```

*Default:* Static case.

```
.B FREQ
1
0.0
```

### .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*:

```
.EXCPRP
5 5 5 5
0 0 0 0
```

Compute the excited state expectation values |\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 with positive integers to generate transition state moments |\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¶

## Control integral contributions¶

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

### .INTFLG¶

Specify what two-electron integrals to include
(default: *.INTFLG* under ***HAMILTONIAN*).

### .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:*

```
.ITRINT
1 1
```

## Control trial vectors¶

### .XQRNRM¶

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

*Default:* Use un-normalized vectors.