# Linking to math libraries¶

## General¶

Dalton requires BLAS and LAPACK libraries. Typically you will want to link to external math (BLAS and LAPACK) libraries, for instance provided by MKL or Atlas.

By default the CMake configuration script will automatically detect these libraries:

```
$ ./setup --blas=auto --lapack=auto # this is the default
```

if you define MATH_ROOT, for instance:

```
$ export MATH_ROOT='/opt/intel/mkl'
```

Do not use full path MATH_ROOT=’/opt/intel/mkl/lib/ia32’. CMake will append the correct paths depending on the processor and the default integer type. If the MKL libraries that you want to use reside in /opt/intel/mkl/10.0.3.020/lib/em64t, then MATH_ROOT is defined as:

```
$ export MATH_ROOT='/opt/intel/mkl/10.0.3.020'
```

Then:

```
$ ./setup [--flags]
$ cd build
$ make
```

The math library detection will attempt to locate libraries based on MATH_ROOT, BLAS_ROOT/LAPACK_ROOT, MKL_ROOT, and MKLROOT.

## Intel/MKL¶

If you compile with Intel compilers and have the MKL library available, you should use the –mkl flag which will automatically link to the MKL libraries (in this case you do not have to set MATH_ROOT). You have to specify whether you want to use the sequential or parallel (threaded) MKL version. For a parallel Dalton runs you should probably link to the sequential MKL:

```
$ ./setup --fc=mpif90 --cc=mpicc --cxx=mpicxx --mkl=sequential
```

For a sequential compilation you may want to link to the parallel MKL:

```
$ ./setup --fc=ifort --cc=icc --cxx=icpc --mkl=parallel
```

The more general solution is to link to the parallel MKL and control the number of threads using MKL environment variables.

## Cray¶

Cray typically provides own optimized BLAS/LAPACK wrappers. For this use the option –cray to disable automatic BLAS/LAPACK detection:

```
$ ./setup --fc=ftn --cc=cc --cxx=CC --cray
```

## Explicitly specifying BLAS and LAPACK libraries¶

If automatic detection of math libraries fails for whatever reason, you can always call the libraries explicitly like here:

```
$ ./setup --blas=/usr/lib/libblas.so --lapack=/usr/lib/liblapack.so
```

Alternatively you can use the –explicit-libs option. But in this case you should disable BLAS/LAPACK detection:

```
$ ./setup --blas=none --lapack=none --explicit-libs="-L/usr/lib -lblas -llapack"
```

## Builtin BLAS and LAPACK implementation¶

If no external BLAS and LAPACK libraries are available, you can use the builtin implementation. However note that these are not optimized and you will sacrifice performance. This should be the last resort if nothing else is available:

```
$ ./setup --blas=builtin --lapack=builtin
```

## LSDalton using ScaLAPACK/Intel/MKL¶

If you compile with Intel compilers and have the MKL library available, you can chose to compile LSDalton using the ScaLAPACK library provided by Intel. In this case you should set the MATH_ROOT environment variable and use the –scalapack flag which will automatically link to the MKL libraries.

You should not use the –mkl flag for this setup:

```
$ ./setup --fc=mpif90 --cc=mpicc --cxx=mpicxx --scalapack
```