# GUT scale condition in SPheno

## General

Using the variable

`ConditionGUTscale = X==Y</code>`

one defines a dynamical condition for the GUT scale in the SPheno evaluation. As soon as SPheno finds that the condition is fulfilled, it stops the RGE running up to higher scales and applies the boundary conditions given in `BoundaryHighScale`

, see Boundary conditions in SPheno. Note, that SPheno assumes that at low scales *X* < *Y* holds.
If no condition is defined a warning is printed and the unification of the first two gauge couplings in the array `Gauge`

is taken as condition.

### Example

A common choice for the GUT scale is the scale at which the two electroweak gauge couplings unify:

`ConditionGUTscale = g1 == g2;`

However, also more complicated relations can be used and it is possible to use also other running parameters then the gauge couplings. Note that `ConditionGUTscale = g2 == g1;`

won't work because SPheno would assume that the GUT scale is already passed at very low energies.

## Using a fixed value for the GUT scale

It is always possible to work with a fixed value for the GUT scale via the Les Houches input file, e.g.

```
Block SPhenoInput # SPheno specific input
....
31 2.000E+16 # fixed GUT scale (-1: dynamical GUT scale)
```

Only with

```
Block SPhenoInput # SPheno specific input
....
31 -1 # fixed GUT scale (-1: dynamical GUT scale)
```

the condition defined by `ConditionGUTscale`

is used to get a dynamical GUT scale.