# QMMM Optimization

QM/MM optimization is based on multi-region optimization methodology and is invoked by

```
task qmmm <qmtheory> optimize
```

The overall algorithm involves alternating optimizations of QM and MM regions until convergence is achieved. This type of approach offers substantial savings compared to direct optimization of the entire system as a whole. In the simplest case of two regions (QM and MM) the algorithm is comprised of the following steps:

- Optimization of the QM region keeping MM region fixed
- Calculation of reduced electrostatic representation for the QM region (e.g. ESP charges)
- Optimization of MM region keeping QM region fixed
- Repeat from Step 1 until converged

The optimization process is controlled by the following keywords:

**region**-**required**keyword which specifies which regions will optimized and in which order.**maxiter**- number of optimizations steps for each region within single optimization pass**ncycles**- number of optimization cycles**density**- electrostatic representation of the QM region during MM optimization**xyz**- output of xyz structure files**convergence**- convergence criteria

Here is an example QM/MM block that provides practical illustration of all these keywords for a generic optimization case where QM molecule(s) are embedded in the solvent

```
qmmm
[region] qm solvent
[maxiter] 10 3000
[ncycles] 5
[density] espfit
[xyz] foo
end
```

We have two regions in the system “qm” and “solvent” and we would like to optimize them both, thus the line

```
[region] qm solvent
```

Our QM region is presumably small and the maximum number of iterations (within a single optimization pass) is set to 10. The solvent region is typically much larger (thousands of atoms) and the maximum number of iterations is set to a much large number 3000:

```
[maxiter] 10 3000
```

We would like to perform a total of 5 optimization passes, giving us a total of 5*10=50 optimization steps for QM region and 5*3000=15000 optimization steps for solvent region:

```
[ncycles] 5
```

We are requesting QM region to be represented by point ESP charges during the solvent optimization:

```
[density] espfit
```

Finally we are requesting that the coordinates of the first region to be saved in the form of numbered xyz files:

```
[xyz] foo
```