.. tip:: All input files can be downloaded: :download:`Files `. mecp ======== .. contents:: :local: This keyword defines the arguments for searching minimum energy crossing point (MECP). .. hint:: When using this keyword, in ``scf...end`` block, you must **ALWAYS** set ``type u``. Options ------------ .. option:: num_steps .. list-table:: :stub-columns: 1 :widths: 5 20 * - Value - An integer * - Default - ``200`` The maximum number of MECP search. .. option:: energy_cov .. list-table:: :stub-columns: 1 :widths: 5 20 * - Value - A real number * - Default - ``1.E-5`` The energy difference convergence threshold. When ``energy_cov``, ``grad_cov``, and ``dr_cov`` are all met, the MECP will be set to be converged. .. option:: grad_cov .. list-table:: :stub-columns: 1 :widths: 5 20 * - Value - A real number * - Default - ``1.E-3`` The gradient convergence threshold. When ``energy_cov``, ``grad_cov``, and ``dr_cov`` are all met, the MECP will be set to be converged. .. option:: dr_cov .. list-table:: :stub-columns: 1 :widths: 5 20 * - Value - A real number * - Default - ``1.E-3`` The displacement convergence threshold. When ``energy_cov``, ``grad_cov``, and ``dr_cov`` are all met, the MECP will be set to be converged. .. option:: orb1 This defines the electronic configuration of state 1. The format is: ``orb num_electrons spin_multiplicity alpha_MO_indices : beta_MO_indices`` There can be arbitrary number of ``orb1``, but all orbitals must be included once and only once. They are the same as ``orb`` in ``scfguess``. See :doc:`scfguess`. .. option:: orb2 This defines the electronic configuration of state 2. The format is: ``orb num_electrons spin_multiplicity alpha_MO_indices : beta_MO_indices`` There can be arbitrary number of ``orb2``, but all orbitals must be included once and only once. They are the same as ``orb`` in ``scfguess``. See :doc:`scfguess`. .. option:: frag1 This defines the fragmentation of state 1. The format is: ``frag num_electrons spin_multiplicity atom_range`` There can be arbitrary number of ``frag1``, but all atoms must be included once and only once. They are the same as ``frag`` in ``scfguess``. See :doc:`scfguess`. .. option:: frag2 This defines the fragmentation of state 1. The format is: ``frag num_electrons spin_multiplicity atom_range`` There can be arbitrary number of ``frag2``, but all atoms must be included once and only once. They are the same as ``frag`` in ``scfguess``. See :doc:`scfguess`. .. HINT:: When ``frag1/2`` and ``orb1/2`` are defined simultaneously, ``orb1/2`` will be considered in priority. Theoretical Background ----------------------- MECP Between Ground and Excited States ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The minimum energy crossing point (MECP) is a point where two potential energy surfaces (PES) of different electronic states cross each other. In the context of quantum chemistry, it is often used to describe the transition between a ground state and an excited state. The MECP can be found by optimizing the geometry of the system while constraining the energy difference between the two states to be zero. Diabatic MECP (dMECP) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Diabatic MECP (dMECP) is a method used to find the transition state for a AB+C=A+BC reaction. It uses the MECP search algorithm to find the minimum energy crossing point between the reactant and product diabatic states. Input Examples ------------------------- Example: MECP between the Singlet and Triplet States of C\ :sub:`6`\ H\ :sub:`5`:sup:`+` ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In this example, we will search the MECP between the singlet and triplet states of C\ :sub:`6`\ H\ :sub:`5`:sup:`+`. The input file is as follows: .. code-block:: bash :linenos: :caption: mecp-1.inp basis def2-svp end mol C 0.00000000 1.39661300 0.00000000 C 1.20950300 0.69830700 0.00000000 C 1.20950300 -0.69830700 0.00000000 C 0.00000000 -1.39661300 0.00000000 C -1.20950300 -0.69830700 0.00000000 C -1.20950300 0.69830700 0.00000000 H 2.15088200 1.24181200 0.00000000 H 2.15088200 -1.24181200 0.00000000 H 0.00000000 -2.48362500 0.00000000 H -2.15088200 -1.24181200 0.00000000 H -2.15088200 1.24181200 0.00000000 end mecp num_steps 200 # The number of search steps. energy_cov 1.E-5 # The energy difference convergence threshold. orb1 40 1 1-109 : 1-109 orb2 40 3 1-109 : 1-109 end scf type u charge +1 spin2p1 1 end task mecp b3lyp end In ``mecp...end`` block, we define the two electronic configurations of the singlet and triplet states using ``orb1`` and ``orb2``. The ``num_steps`` is set to ``200``, and the energy convergence threshold is set to ``1.E-5``. The format of ``orb1`` and ``orb2`` is the same as that in :doc:`scfguess`. For example, ``orb1 40 1 1-109 : 1-109`` means that the first state has 40 electrons, a spin multiplicity of 1, and the alpha and beta MOs are from indices 1 to 109. The same applies to ``orb2``. The singlet-triplet MECP is saved to ``mecp-1-mecp.xyz``. The optimized process is saved in ``mecp-1-mecp-traj.xyz``, which can be shown using `Qbics-MolStar `_: .. figure:: figs/mecp-5.gif Example: Search Transition State of Iodine Atom Transfer Reaction ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In this section, we want to search the transition state of the following reaction: .. figure:: figs/mecp-1.jpg This is a AB+C=A+BC type reaction. The largest advantage of dMECP algorithm is that the transition state is searched using energy coordinate instead of geometry coordinate, so it is not too sensitive to the initial guess. The input file is as follows. Note that for iodine, we have used pseudopotential with ``pseudopotential`` keyword, ``def2-ecp``. .. code-block:: bash :linenos: :caption: mecp-2.inp basis def2-svp end pseudopotential def2-ecp end scf type u charge 0 spin2p1 2 end mol C -0.29500 -0.00002 -0.00017 C 1.10016 -0.00002 -0.00017 C 1.79770 1.20773 -0.00017 C 1.10004 2.41624 -0.00137 C -0.29478 2.41616 -0.00185 C -0.99238 1.20795 -0.00085 H -0.84476 -0.95234 0.00028 H 1.64967 -0.95253 0.00114 I 3.87770 1.20788 0.00103 H 1.65024 3.36838 -0.00143 H -0.84490 3.36844 -0.00280 H -2.09199 1.20814 -0.00103 Si 6.95770 1.20810 0.00280 C 7.59353 0.30695 1.56221 C 7.59533 0.30859 -1.55681 C 7.59423 3.00892 0.00412 H 7.23873 -0.70024 -1.55754 H 7.23913 0.81342 -2.43040 H 8.66533 0.30866 -1.55619 H 8.66353 0.30703 1.56282 H 7.23632 0.81087 2.43592 H 7.23694 -0.70188 1.56147 H 8.66423 3.00899 0.00563 H 7.23876 3.51349 -0.86992 H 7.23629 3.51309 0.87738 end mecp num_steps 200 energy_cov 1.E-5 frag1 0 1 1-12 frag1 0 2 13-25 frag2 0 2 1-8 10-12 frag2 0 1 9 13-25 end task mecp b3lyp end The structure given in ``mol...end`` is shown below: .. figure:: figs/mecp-2.jpg We can see that the initial structure is a complex that two reactants are simply putting together. Now, according to the reactant and product species, we can define ``frag1`` and ``frag2`` in ``mecp...end`` option block. The definitions are shown below, which is self-explanatory and in line with chemical intuition: .. figure:: figs/mecp-3.jpg In dMECP method, only a single geometry is needed. Of course, the closer it approaches the transition state, the better. The optimized structure ``mecp-2-mecp.xyz`` is shown below: .. figure:: figs/mecp-4.jpg