.. _saddle: =========== Saddle =========== One and two-sided saddle point search methods are implemented in FLAME. Note that not all parameters apply to all available saddle point search methods when **task** is set to ``saddle``. saddle parameters ================================= general ``saddle`` parameters ------------------------------------------ **method**: (string) Method for the saddle point search. default: ``No default value.`` options: ``bar_saddle``: The bar-saddle method from Ref. :cite:`schaefer_minima_2014`. ``dimer``: The dimer method from Ref. :cite:`henkelman_dimer_1999`. ``splined_saddle``: The splined saddle method from Ref. :cite:`ghasemi_an_2011`. ``bar_saddle`` parameters ----------------------------------- **dbar**: (real) The length of the bar/dimer in the bar-saddle method. In units of Bohr in configurational space. default: ``1.d-1`` **nstep**: (integer) Maximal number of steps for the saddle search. default: ``1000`` **stepsize**: (real) The initial step size for the saddle point search iterations. Will be adjusted based on a a feedback mechanism. Arbitratry units. default: ``20.d0`` **bar_contract**: (logical) Activates the contraction of the bar size during the optimization. The final bar size can be set with **dbar_contracted**. The contraction occurs gradually over **nstep_contract** iterations. default: ``True`` **dbar_contracted**: (real) The final bar size after contraction. In units of Bohr in configurational space. default: ``1.d-3`` **nstep_contract**: (integer) Number of iterations to contract the bar from the initial value **dbar** to the final length **dbar_contracted**. default: ``40`` **fnrmtol_coarse**: (real) The force norm tolerance that has to be reached to commence the contraction of the bar. Units in Ha/Bohr. default: ``1.d-2`` **fnrmtol_contracted**: (real) The force norm tolerance for the convergence of the final, contracted bar. Units in Ha/Bohr. default: ``5.d-4`` ``dimer`` parameters ----------------------------------- **list_random_displace**: (list of integers) List of atomic indices that are displaced randomly before initiating the saddle optimization in the dimer method. default: ``No default value.`` **ampl**: (real) The amplitude of the random displacement applied to the atoms listed by the key **list_random_displace**. In units of Bohr in configurational space. default: ``No default value.`` **dimsep**: (real) The dimer separation in the dimer method. No default value given, must be set when the dimer method is invoked. In units of Bohr in configurational space. default: ``No default value.`` ``splined_saddle`` parameters ----------------------------------- **np_splsad**: (integer) Number of anchor points. **np_splsad**-1 is the number of moving anchor points in the splined saddle method. Note: with the default value of ``2``, the splined saddle will only have one moving achor point. default: ``2`` **np_neb**: (integer) Number of NEB images. **np_neb**-1 is the number of moving images in the nudged elastic band (NEB) method. Note: with the default value of ``2``, the NEB will run with only one moving image. default: ``2`` **ns2**: (integer) Number of extra points added to the number of anchor points in order to assist the maximization process, the inner loop of the splined saddle method. default: ``0`` **vdtol**: (real) The convergence criterion in the maximization process in the inner loop of the splined saddle method. Units in Ha. default: ``1.d-1`` **htol**: (real) The smallest value of the normalized pathway parameter between two neighboring points in the maximization process of the splined saddle method. Arbitrary units. default: ``2.d-2`` **alphax**: (real) The step size of the optimizer in the NEB and splined saddle methods. Arbitrary units. default: ``5.d-1`` **docineb**: (string) Activates the climbing image NEB. default: ``no`` options: ``no``: NEB without climbing image ``yes``: NEB with climbing image **doneb**: (string) Activates NEB calculations. default: ``No default value.`` options: ``no``: No NEB calcualtion is performed ``yes``: NEB calculation is performed **pickbestanchorpoints**: (string) Activates an automated selection of favored anchor points. This feature is not well tested and we recommend setting this parameter to ``no``. default: ``No default value.`` options: ``no``: anchor points are distributed uniformly in the beginning of simulation ``yes``: anchor points are initially selected to favor higher energy points based on estimates obtained by an interpolation. **runstat**: (string) Determines whether a new or a restart run is performed. default: ``new`` options: ``new``: New run, the NEB images or splined saddle anchor points are initialized at the beginning of the run. ``restart``: Restart run, the NEB images or splined saddle anchor points are read from a file. Restart runs have not yet been well tested, so we currently do not recommend using it. **typintpol**: (string) The type of interpolation in the maximization process of the splined saddle method. default: ``cubic`` options: ``cubic``: Natural cubic splines. ``quintic``: A spline using fifth-order polynomials. Rather unstable except for simple pathways. **fcalls_max**: (integer) The maximum number of calls to force evaluation. default: ``100`` **fmaxtol_splsad**: (real) The convergence criterion for the saddle optimization in the splined saddle method. Units in Ha/Bohr. default: ``2.d-4`` **fmaxtol_neb**: (real) The convergence criterion for the saddle optimization in the NEB method. Units in Ha/Bohr. default: ``2.d-2`` **opt_method**: The optimization method used in the saddle point search when using NEB or the splined saddle method. default: ``SD`` options: ``SD``: The steepest descent method. ``BFGS``: The Broyden–Fletcher–Goldfarb–Shanno (BFGS) method.