number of cycles, ORCA will not go on to do a frequency calculation.Ĭhoose a good starting geometry.
Note that in ORCA 4.0, if a geometry optimization does not converge within the Max. The default convergence criteria (NormalOpt) are: TolE=5e-6, TolRMSG=1e-4, TolMaxG=3e-4, TolRMSD=2e-3, TolMaxD=4e-3. TightSCF convergence criteria is enforced by default in ORCA for geometry optimizations (reduces numerical noise in the gradient). The initial Hessian is by default an approximate model Hessian by Almlöf (other possibilities are model Hessians by Lindh and Schlegel, a diagonal Hessian or a previously calculated Hessian).
Chemistry geometry calculator update#
ORCA, by default uses a Quasi-Newton optimiser using the BFGS update (Powell and Bofill also available) and the optimization is carried out in redundant internal coordinates. See GFN1-xTB paper, GFN2-xTB paper and a study on large TM complexes with GFN-xTB methods. The GFN-xTB tightbinding methods (available in ORCA 4.2). ĭFT-3c methods such as: r2SCAN-3c, B97-3c and PBEh-3c which It's even cheaper than a DFT calculation and more robust than a standard AM1/PM3 semiempirical calculation (but not as fast). The HF-3c method which is a minimal-basis HF calculation, corrected for dispersion, BSSE and basis set incompleteness. Sometimes NDDO-type semiempirical methods can be useful for pre-optimizations of closed-shell organic molecules (typically not for transition metal compounds) but these methods are not as robust as DFT methods.Īlternative methods for cheap (but often accurate) geometry optimizations (or at least pre-optimizations) come from the Grimme group: RI-MP2 optimizations (MP2 or SCS-MP2) is another option for main-group elements (MP2 is a very robust method for organic molecules) while for transition-metal chemistry, DFT optimizations are generally preferred. The PBE0 functional with a dispersion correction and a triple-zeta basis is probably the most accurate all-round method for geometry optimizations of molecules from the whole periodic table (note that open-shell 3d transition metal systems might be an exception to this). via electron correlation WFT methods) and in practice only possible for small systems. It's typically a considerable effort to get much better geometries than those obtained from DFT calculations (i.e.
Geometry optimizations are usually performed at the DFT level as DFT has a very favorable cost-accuracy ratio and because analytical gradients are available for most functionals.
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