.. tip::
    All input and output files can be downloaded :download:`here <_static/geoopt/geoopt.zip>`.

Geometry Optimization  
============================

Geometry optimization intends to find a minimum on the potential energy surface from a starting geometry. All energy calculation methods, including quantum mechanics (QM), molecular mechanics (MM), and QM/MM, can be used for geometry optimization, as long as you change ``energy`` to ``opt`` in the input files.

Three Optimizations
--------------------------

QM Geometry Optimization
++++++++++++++++++++++++++++++

For water in :doc:`qmenergy`, the geometry optimization is:


.. code-block:: 
    :caption: water.inp

    basis
        def2-tzvp
    end
      
    scf
        charge  0
        spin2p1 1
        type    R
    end
   
    grimmedisp
        type bj
    end

    mol
       O  0.    0.00000000   -0.11081188
       H  0.   -0.78397589    0.44324751
       H  0.    0.78397589    0.44324751
    end
    
    task
        opt b3lyp # Just change energy to opt.
    end

After optimization, you will find two additional files:

- ``water-opt.xyz`` The last optimized structure. It is updating during the optimization.
- ``water-opt-traj.xyz`` All intermediate structures during the optimization.


MM Geometry Optimization
++++++++++++++++++++++++++++++

The protein example in :doc:`mmenergy` can be easily changed for geometry optimization:


.. code-block:: 
    :caption: protein.inp

    charmm
        parameters par_all36m_prot.prm toppar_water_ions.str
        topology   step2_solvator.psf
        scaling14  1.0
        rcutoff    12.0
        rswitch    10.0                       
        use_PBC        
        Lbox 64 64 64  
        electrostatic pme
        PMEk   64 64 64 
    end
    
    mol
        step2_solvator.pdb
    end
    
    task   
        opt charmm
    end
    
QM/MM Geometry Optimization
++++++++++++++++++++++++++++++

Both examples in :doc:`qmmmenergy` can be easily changed for geometry optimization, as long as you change ``energy`` to ``opt``. For example:


.. code-block:: 
    :caption: protein-qmmm.inp
    
    charmm
        parameters par_all36m_prot.prm
        topology   step1_pdbreader.psf
        scaling14  1.0
        rcutoff    12.0
        rswitch    13.0
    end
    
    basis
        def2-svp
    end
      
    scf
        charge  0
        spin2p1 1
        type    R
    end
    
    grimmedisp
        type bj
    end
    
    qmmm
        qm_region  779 792-815
    end
    
    mol
        step1_pdbreader.pdb
    end
    
    task   
        opt b3lyp/charmm
    end
    

Some Options
--------------------------

There are some options for geometry optimization. For example, the following example:

.. code-block:: 
    :caption: c3h8o2.inp

    basis
        def2-svp
    end
      
    scf
        charge  0
        spin2p1 1
        type    R
    end
    
    grimmedisp
        type bj
    end

    opt
       num_steps   1000 
       epsilon     1.E-5
       fix         1-9
    end
    
    mol
          C                 -4.08368628    0.86864405    0.00000000
          H                 -3.72701344    1.37304224   -0.87365150
          H                 -5.15368628    0.86865724    0.00000000
          C                 -3.57037057   -0.58328810    0.00000000
          H                 -3.92704565   -1.08768708   -0.87365013
          H                 -3.92704123   -1.08768606    0.87365253
          C                 -2.03037057   -0.58330630   -0.00000339
          H                 -1.67371614   -1.59211630   -0.00000091
          H                 -1.67370018   -0.07891023   -0.87365712
          O                 -3.60701136    1.54274631    1.16759033
          H                 -4.11695915    2.34433532    1.30546096
          O                 -1.55369311    0.09080013    1.16758349
          H                 -2.04104440   -0.21550578    1.93587082
    end
    
    task   
        opt b3lyp
    end
    
The geometry optimization control block is ``opt ... end``:

- ``num_steps`` The maximum number of geometry steps. Default is ``1000``.
- ``epsilon`` A convergence label. Usually, you do not need to change it.
- ``fix`` The atoms that you want to fix during geometry optimization.

In the above example, we fix the alkyl chain and only relax the hydroxyl groups. The final optimization result can be seen in ``c3h8o2-opt.xyz`` and ``c3h8o2-opt-traj.xyz``. The animation of the optimization is shown below. Obviously, atoms ``1-9`` have been fixed.

.. image:: _static/geoopt/p1.gif
    :align: center