Copyright (C) 2013-2016 Sebastian Wouters sebastianwouters@gmail.com

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

CheMPS2 is a scientific library which contains a spin-adapted implementation of the density matrix renormalization group (DMRG) for ab initio quantum chemistry. This wavefunction method allows to obtain numerical accuracy in active spaces beyond the capabilities of full configuration interaction (FCI), and allows to extract the 2-, 3-, and 4-particle reduced density matrices (2-, 3- and 4-RDM) of the active space.

For general active spaces up to 40 electrons in 40 orbitals can be handled with DMRG, and for one-dimensional active spaces up to 100 electrons in 100 orbitals. The 2-RDM of these active spaces can also be easily extracted, while the 3- and 4-RDM are limited to about 28 orbitals.

When the active space size becomes prohibitively expensive for FCI, DMRG can be used to replace the FCI solver in the complete active space self consistent field (CASSCF) method and the corresponding complete active space second order perturbation theory (CASPT2). The corresponding methods are called DMRG-SCF and DMRG-CASPT2, respectively. For DMRG-SCF the active space 2-RDM is required, and for DMRG-CASPT2 the active space 4-RDM.

CheMPS2 is designed for high-performance computers, with a hybrid parallelization for mixed distributed and shared memory architectures, realized with the Message Passing Interface (MPI) and the Open Multi-Processing (OpenMP) API.

The CheMPS2 library can be interfaced with quantum chemistry codes which can handle R(O)HF calculations and molecular orbital matrix elements. This has been done for psi4 and pyscf, as described in the user manual. Usage of the library is illustrated in the c++ and python tests.

The CheMPS2 binary allows to perform DMRG-SCF and DMRG-CASPT2 calculations based on input files. Molecular orbital matrix elements should then be provided in FCIDUMP format. Usage of the binary is illustrated in the binary example.

Information on CheMPS2 can be found in the following documents:

1. publications
2. user manual
3. doxygen html output

The user manual contains elaborate information on

• the installation of CheMPS2
• the DMRG, DMRG-SCF, and DMRG-CASPT2 algorithms
• the symmetries which are exploited in CheMPS2
• how to generate matrix elements with plugins to psi4
• how to perform DMRG, DMRG-SCF, and DMRG-CASPT2 calculations
• the interfaces to psi4 and pyscf

To acknowledge CheMPS2, please cite

1. S. Wouters, W. Poelmans, P.W. Ayers and D. Van Neck, CheMPS2: a free open-source spin-adapted implementation of the density matrix renormalization group for ab initio quantum chemistry, Computer Physics Communications 185 (6), 1501-1514 (2014), doi:10.1016/j.cpc.2014.01.019

    @article{CheMPS2cite1,
        author = {Sebastian Wouters and Ward Poelmans and Paul W. Ayers and Dimitri {Van Neck}},
        title = {CheMPS2: a free open-source spin-adapted implementation of the density matrix renormalization group for ab initio quantum chemistry},
        journal = {Computer Physics Communications},
        year = {2014},
        volume = {185},
        number = {6},
        pages = {1501-1514},
        doi = {10.1016/j.cpc.2014.01.019}
    }
   

2. S. Wouters and D. Van Neck, The density matrix renormalization group for ab initio quantum chemistry, European Physical Journal D 68 (9), 272 (2014), doi:10.1140/epjd/e2014-50500-1

    @article{CheMPS2cite2,
        author = {Sebastian Wouters and Dimitri {Van Neck}},
        title = {The density matrix renormalization group for ab initio quantum chemistry},
        journal = {European Physical Journal D},
        year = {2014},
        volume = {68},
        number = {9},
        pages = {272},
        doi = {10.1140/epjd/e2014-50500-1}
    }
   

3. S. Wouters, T. Bogaerts, P. Van Der Voort, V. Van Speybroeck and D. Van Neck, Communication: DMRG-SCF study of the singlet, triplet, and quintet states of oxo-Mn(Salen), Journal of Chemical Physics 140 (24), 241103 (2014), doi:10.1063/1.4885815

    @article{CheMPS2cite3,
        author = {Sebastian Wouters and Thomas Bogaerts and Pascal {Van Der Voort} and Veronique {Van Speybroeck} and Dimitri {Van Neck}},
        title = {Communication: DMRG-SCF study of the singlet, triplet, and quintet states of oxo-Mn(Salen)},
        journal = {Journal of Chemical Physics},
        year = {2014},
        volume = {140},
        number = {24},
        pages = {241103},
        doi = {10.1063/1.4885815}
    }
   

4. S. Wouters, V. Van Speybroeck and D. Van Neck, DMRG-CASPT2 study of the longitudinal static second hyperpolarizability of all-trans polyenes, Preprint (2016), arXiv:1605.05526

    @article{CheMPS2cite4,
        author = {Sebastian Wouters and Veronique {Van Speybroeck} and Dimitri {Van Neck}},
        title = {DMRG-CASPT2 study of the longitudinal static second hyperpolarizability of all-trans polyenes},
        journal = {arXiv:1605.05526},
        year = {2016},
        url = {http://arxiv.org/abs/1605.05526}
    }
   

CheMPS2 can be built with CMake and depends on

• BLAS
• LAPACK
• HDF5 (Hierarchical Data Format Release 5)

It is parallelized for shared memory architectures with the Open Multi-Processing (OpenMP) API and for distributed memory architectures with the Message Passing Interface (MPI). A hybrid combination of both parallelization strategies is supported.

It is advised to clone the CheMPS2 git repository from github. In your terminal, run:

> cd /sourcefolder
> git clone 'https://github.com/sebwouters/chemps2'
> cd chemps2

That way, future updates and bug fixes can be easily pulled in:

> cd /sourcefolder/chemps2
> git pull

The files

/sourcefolder/chemps2/CMakeLists.txt
/sourcefolder/chemps2/CheMPS2/CMakeLists.txt
/sourcefolder/chemps2/tests/CMakeLists.txt
/sourcefolder/chemps2/sphinx/CMakeLists.txt

provide a minimal compilation. In your terminal, run:

> cd /sourcefolder/chemps2
> mkdir build
> cd build

CMake generates makefiles based on the user’s specifications:

> CXX=option1 cmake .. -DMKL=option2 -DCMAKE_INSTALL_PREFIX=/option3 -DWITH_MPI=option4

1. Option1 is the c++ compiler; typically g++, icpc, or clang++ on Linux. It is advised to use the intel compiler if available.
2. Option2 can be ON or OFF and is used to switch on the intel math kernel library.
3. /option3 is the prefix of the installation directory; typically /usr or /usr/local on Linux. On my computer, libchemps2 is then installed in /option3/lib/x86_64-linux-gnu, the headers in /option3/include/chemps2, and the binary in /option3/bin/chemps2.
4. Option4 can be ON or OFF and is used to switch on the possibility to compile with MPI. Please note that the compiler should then provide mpi.h. Option1 should hence be the mpic++ compiler; typically mpic++ or mpiicpc on Linux. It is advised to use the intel compiler if available.

If one or more of the required libraries are not found, use the command

> CMAKE_INCLUDE_PATH=option5 CMAKE_LIBRARY_PATH=option6 CXX=option1 cmake .. -DMKL=option2 -DCMAKE_INSTALL_PREFIX=/option3 -DWITH_MPI=option4

instead, where option5 and option6 are respectively the missing colon-separated include and library paths:

CMAKE_INCLUDE_PATH=/my_libs/lib1/include:/my_libs/lib2/include
CMAKE_LIBRARY_PATH=/my_libs/lib1/lib:/my_libs/lib2/lib

For operating systems based on debian, the HDF5 headers are located in the folder /usr/include/hdf5/serial. If CMake complains about the HDF5 headers, try to pass it with the option -DHDF5_INCLUDE_DIRS=/usr/include/hdf5/serial.

To compile, run:

> make

To install, run:

> make install

For non-standard installation directories, please remember to append the library path to LD_LIBRARY_PATH in your .bashrc.

To test libchemps2 for compilation without MPI, run:

> cd /sourcefolder/chemps2/build
> make test

To test libchemps2 for compilation with MPI, run:

> cd /sourcefolder/chemps2/build/tests
> OMP_NUM_THREADS=YYY mpirun -np ZZZ ./test1
> OMP_NUM_THREADS=YYY mpirun -np ZZZ ./test2
...

YYY specifies the number of threads per process and ZZZ the number of processes. Note that the tests are too small to see (near) linear scaling with the number of cores, although improvement should still be noticeable.

To test the chemps2 binary for compilation without MPI, run:

> man /sourcefolder/chemps2/chemps2.1
> cd /sourcefolder/chemps2/build/CheMPS2
> ./chemps2 --help
> cp /sourcefolder/chemps2/tests/test14.input .
> sed -i "s/path\/to/sourcefolder\/chemps2\/tests\/matrixelements/" test14.input
> cat test14.input
> ./chemps2 --file=test14.input

Note that when you use the CASPT2 checkpoint, and want to restart a calculation at a later point, you should

1. switch the option SCF_ACTIVE_SPACE to I
2. remove the CheMPS2_DIIS.h5 checkpoint

in order to ensure that exactly the same orbitals are used in the different runs.

To test the chemps2 binary for compilation with MPI, prepend the binary with:

> OMP_NUM_THREADS=YYY mpirun -np ZZZ ./chemps2 --file=test14.input

PyCheMPS2 is a python interface to libchemps2, for compilation without MPI. It can be built with Cython. The installation is independent of CMake and assumes that you have installed the CheMPS2 library with make install. For non-standard installation directories of CheMPS2, please remember to append the library path to LD_LIBRARY_PATH in your .bashrc. In addition, the include path should be appended to CPATH:

> export CPATH=${CPATH}:/option3/include

where /option3 is the option provided to CMake with -DCMAKE_INSTALL_PREFIX=/option3 above. For operating systems based on debian, the HDF5 headers are located in the folder /usr/include/hdf5/serial. If it was explicitly passed to CMake, it should also be appended to CPATH:

> export CPATH=${CPATH}:/option3/include:/usr/include/hdf5/serial

The python wrapper can be installed with:

> cd /sourcefolder/chemps2/PyCheMPS2
> python setup.py build_ext -L ${LD_LIBRARY_PATH}
> python setup.py install --prefix=/option3

On my machine, the python wrapper is installed to the folder /option3/lib/python2.7/site-packages, but the folder lib and the distribution of python can vary.

Compilation of PyCheMPS2 occurs by linking to the c++ library in the installation directory. The installation of PyCheMPS2 will fail if that library is not properly installed. If you have pulled a newer version of CheMPS2, please remember to reinstall the c++ library first, before reinstalling PyCheMPS2!

When libchemps2 has been compiled without MPI, PyCheMPS2 can be tested by running (remember that the python site-packages folder can vary):

> cd /sourcefolder/chemps2/PyCheMPS2/tests
> export PYTHONPATH=${PYTHONPATH}:/option3/lib/python2.7/site-packages
> python test1.py
> python test2.py
...

If you compiled the c++ library with -DMKL=ON, you might get the error

Intel MKL FATAL ERROR: Cannot load libmkl_avx.so or libmkl_def.so.

This issue of using Intel’s MKL inside python is known and reported. To get the python tests to run, you can set the variable LD_PRELOAD in order to preload libmkl_rt.so. On my system, this is done with

> export LD_PRELOAD=/opt/intel/mkl/lib/intel64/libmkl_rt.so

The python tests do exactly the same thing as the c++ tests above, and illustrate the usage of the python interface to libchemps2. The tests should end with a line stating whether or not they succeeded. Note that the tests are too small to see (near) linear scaling with the number of cores, although improvement should still be noticeable.

The files in the CheMPS2 library, as well as the files required to perform test runs, can be found in FILES.md.