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https://github.com/piecuch-group/cct3

CCT3: A PSI4 plugin which performs active-space coupled-cluster CCSDt calculations and which can determine non-iterative corrections to CCSDt defining the CC(t;3) approach.
https://github.com/piecuch-group/cct3

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CCT3: A PSI4 plugin which performs active-space coupled-cluster CCSDt calculations and which can determine non-iterative corrections to CCSDt defining the CC(t;3) approach.

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README 

        
CC(t;3) PSI4 Plugin 

===================

.. image:: https://travis-ci.org/piecuch-group/cct3.svg?branch=dev

    :target: https://travis-ci.org/piecuch-group/cct3



The CCT3 plugin to PSI4 [1]_ is capable of executing a number of closed-

and open-shell coupled-cluster (CC) calculations with up to triply excited (T3)

clusters. Among them is the active-space CC approach abbreviated as CCSDt

[2]_, [3]_, [4]_, [5]_, which approximates full CCSDT by selecting the dominant T\ :sub:`3` amplitudes

via active orbitals, and the CC(t;3) method, which corrects the CCSDt energies

for the remaining, predominantly dynamical, triple excitations that have not

been captured by CCSDt [6]_, [7]_. The CC(t;3) approach belongs to a larger family

of methods that rely on the generalized form of biorthogonal moment expansions

defining the CC(P;Q) formalism [6]_, [7]_.



The CCSDt method alone is already very useful, since it can reproduce

electronic energies of the near-CCSDT quality at a small fraction of the

computational cost, while accurately describing selected multireference

situations, such as single bond breaking. CC(t;3) improves the CCSDt energetics

even further, being practically as accurate as full CCSDT for both relative and

total electronic energies, at a cost which is essentially the same at that of

CCSDt. The systematic convergence of the CCSDt and CC(t;3) calculations toward

CCSDT should be emphasized here too. For example, CCSDt becomes full CCSDT when

all orbitals used to select T3 amplitudes are active. The same applies to

CC(t;3) (when all orbitals used to select T3 amplitudes are active, the triples

correction to CCSDt becomes zero).



The CCT3 plugin can also be used to run CCSD and CR-CC(2,3) calculations. This

can be done by making the active orbital set, which is defined by the user in

the input, empty, since in this case CCSDt = CCSD and CC(t;3) = CR-CC(2,3). We

recall that CR-CC(2,3) is a completely renormalized triples correction to CCSD,

which improves the results obtained with the conventional CCSD(T) approach

without resorting to any multireference concepts and being at most twice as

expensive as CCSD(T) [8]_, [9]_, [10]_.



Please note that UHF/UKS references are not supported by CCT3.



Build

-----



To compile this plugin, a working version of PSI4 version 1.1 or greater is

required. The easiest way to get a working copy is via the `conda

`_ or `anaconda

`_ environment (more on this `here

`_). To use it,

first create a new python 3 environment and activate it via:



.. code-block:: bash



   $ conda create -n p4env python=3.7 psi4 psi4-dev -c psi4 -c psi4/label/dev

   $ source activate p4env



Next, get the source code for the CC(t;3) plugin and compile it.



Compiling using the highest optimization level (-O3) is recommended for best

performance, but the compiler may use large amounts of memory (16+ GiB) during the

build.

If that is a problem, lower optimization levels may be used, but the speed of the

resulting CCT3 binary may suffer.



If the machine has at least 32 GiB of RAM, you should have no problem executing

the following lines:



.. code-block:: bash



   $ git clone https://github.com/piecuch-group/cct3

   $ cd cct3

   $ `psi4 --plugin-compile` -DCMAKE_Fortran_FLAGS="-O3"

   $ make

   $ make install

   

Once this step is done, you should have a working copy of the plugin. You can

run a test example with:



.. code-block:: bash



   $ psi4 examples/H8-0.1.dat

   

**Note:** if you run into trouble importing the ``cct3`` module, try adding

the path of the directory containing the ``cct3`` folder to your ``PYTHONPATH``.

   

.. code-block:: bash



   $ export PYTHONPATH="/path/to/cct3/parent/folder:$PYTHONPATH"



Run

---



In order to run a CCSD, CR-CC(2,3), CCSDt, or CC(t;3) calculation, the following

options have to be set within

the scheme



::



   set psi4-cct3 {

      option value

      ...

   }



froz

   Number of frozen core molecular orbitals.

act_occ

   Number of active occupied molecular orbitals counting from the Fermi level

   down (e.g. HOMO, HOMO-1, HOMO-2, etc.).

act_unocc

   Number of active unnocupied molecular orbitals counting from the Fermi level

   up (e.g. LUMO, LUMO+1, LUMO+2, etc.).

etol

   Energy convergence tolerance given as 10^-ETOL. Default is 10^-7

max_iter

   Maximum number of iterations. Default is 100.

keep_amps

   If true, write down the converged cluster amplitudes to the file

   ``amplitudes.moe``.

calc_type

   Can be set to ``CCSD``, ``CR-CC``, ``CCSD3A``, or ``CCT3``. These options invoke CCSD, CR-CC(2,3), CCSDt, and CC(t;3) calculations, respectively. It not specified, the default is CCSD.



References

----------



.. [1] \J.E. Deustua, J. Shen, P. Piecuch, "CCT3: A PSI4 Plugin Which Performs Active-Space Coupled-Cluster CCSDt Calculations and Which Can Determine Noniterative Corrections to CCSDt Defining the CC(t;3) Approach."

.. [2] \P. Piecuch, "Active-Space Coupled-Cluster Methods," *Mol. Phys.* **108**, 2987-3015 (2010). DOI: http://dx.doi.org/10.1080/00268976.2010.522608.



.. [3] \N. Oliphant and L. Adamowicz, "The Implementation of the Multireference Coupled-Cluster Method Based on the Single-Reference Formalism," *J. Chem. Phys.* **96**, 3739-3744 (1992). https://doi.org/10.1063/1.461878.

.. [4] \P. Piecuch, N. Oliphant, and L. Adamowicz, "A State-Selective Multi-Reference Coupled-Cluster Theory Employing the Single-Reference Formalism," *J. Chem. Phys.* **99**, 1875-1900 (1993). DOI: http://dx.doi.org/10.1063/1.466179.

.. [5] \P. Piecuch, S.A. Kucharski, and R.J. Barlett, "Coupled-Cluster Methods with Internal and Semi-Internal Triply and Quadruply Excited Clusters: CCSDt and CCSDtq Approaches," *J. Chem. Phys.* **110**, 6103-6122 (1999). DOI: http://dx.doi.org/10.1063/1.478517.

.. [6] \J. Shen and P. Piecuch, "Biorthogonal Moment Expansions in Coupled-Cluster Theory: Review of Key Concepts and Merging the Renormalized and Active-Space Coupled-Cluster Methods," *Chem. Phys.* **401**, 180-202 (2012). DOI: http://dx.doi.org/10.1016/j.chemphys.2011.11.033.

.. [7] \J. Shen and P. Piecuch, "Combining Active-Space Coupled-Cluster Methods with Moment Energy Corrections via the CC(P;Q) Methodology, with Benchmark Calculations for Biradical Transition States," *J. Chem. Phys.* **136**, 144104-1 - 144104-16 (2012). DOI: http://dx.doi.org/10.1063/1.3700802.

.. [8] \P. Piecuch and M. Wloch, "Renormalized Coupled-Cluster Methods Exploiting Left Eigenstates of the Similarity-Transformed Hamiltonian," *J. Chem. Phys.* **123**, 224105-1 - 224105-10 (2005). DOI: http://dx.doi.org/10.1063/1.2137318.

.. [9] \P. Piecuch, M. Wloch, J.R. Gour, and A. Kinal, "Single-Reference, Size-Extensive, Non-Iterative Coupled-cluster Approaches to Bond Breaking and Biradicals," *Chem. Phys. Lett.* **418**, 467-474 (2006). DOI: http://dx.doi.org/10.1016/j.cplett.2005.10.116.

.. [10] \M. Wloch, J.R. Gour, and P. Piecuch, "Extension of the Renormalized Coupled-Cluster Methods Exploiting Left Eigenstates of the Similarity-Transformed Hamiltonian to Open-Shell Systems: A Benchmark Study," *J. Phys. Chem. A* **111**, 11359-11382 (2007). DOI: http://dx.doi.org/10.1021/jp072535l.



Development Team

----------------



| Dr. J. Emiliano Deustua

| COO and Co-founder, Examol

| e-mail: [email protected]

|

| Dr. Jun Shen

| Senior Research Associate, Department of Chemistry, Michigan State University

| e-mail: [email protected]

|

| Professor Piotr Piecuch

| University Distinguished Professor and Michigan State University Foundation Professor, Department of Chemistry, Michigan State University  

| Adjunct Professor, Department of Physics and Astronomy, Michigan State University