# 3.3.18. MCLR — A Program for Linear Response Calculations¶

MCLR computes response calculations on single and multiconfigurational SCF wave functions. One of the basic uses of MCKINLEY and MCLR is to compute analytical Hessians (vibrational frequencies, IR intensities, etc). MCLR can also calculate the Lagrangian multipliers for a MCSCF state included in a state average optimization and construct the effective densities required for analytical gradients of such a state. The use of keyword RLXRoot in the RASSCF program is required. In both cases the explicit request of executing the MCLR module is not required and will be automatic. We postpone further discussion about MCLR to section 3.3.16.

It follows an example of how to optimize an excited state from a previous State-Average (SA) CASSCF calculation.

```
&GATEWAY
Title= acrolein minimum optimization in excited state 2
Coord=$MOLCAS/Coord/Acrolein.xyz
Basis= sto-3g
Group=NoSym
>>> Do while
&SEWARD
&RASSCF
Title= acrolein
Spin= 1; nActEl= 6 0 0; Inactive= 12; Ras2= 5
CiRoot= 3 3 1
Rlxroot= 2
&SLAPAF
>>> EndDo
```

The root selected for optimization has been selected here with the keyword Rlxroot in RASSCF, but it is also possible to select it with keyword SALA in MCLR.

Now if follows an example as how to compute the analytical hessian for the lowest state of each symmetry in a CASSCF calculation (SCF, DFT, and RASSCF analytical Hessians are also available).

```
&GATEWAY
Title=p-benzoquinone anion. Casscf optimized geometry.
Coord = $MOLCAS/Coord/benzoquinone.xyz
Basis= sto-3g
Group= X Y Z
&SEWARD
&RASSCF
TITLE=p-benzoquinone anion. 2B3u state.
SYMMETRY=2; SPIN=2; NACTEL=9 0 0
INACTIVE=8 0 5 0 7 0 4 0
RAS2 =0 3 0 1 0 3 0 1
&MCKINLEY; Perturbation=Hessian
```

The MCLR is automatically called after MCKINLEY and it is not needed in the input.

## 3.3.18.1. MCLR program — Basic and Most Common Keywords¶

- SALA
- Root to relax in geometry optimizations
- ITER
- Number of iterations