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9 Geometry optimization

Geometry optimizations are invoked by the optg input directive, which must follow the input of the method for which the optimization is to be performed. For instance, optimization of the geometry for formaldehyde at the MP2 level is performed with the following input

***,formaldehyde print,basis,orbitals !this is optional: print the basis set and the !occupied orbitals angstrom geometry= !define the nuclear coordinates C O , C , rco H1 , C , rch , O , hco H2 , C , rch , O , hco , H1 , 180

rco=1.182 Ang rch=1.102 Ang hco=122.1789 Degree

basis=vdz !Select basis set hf !Perform HF calculation mp2 !Perform MP2 calculation optg !Optimize geometry for MP2

Optimizations use analytical energy gradients if available; otherwise the gradients are computed by finite differences. Presently, analytical energy gradients are available for the following methods.

hf

Closed and open-shell spin restricted Hartree-Fock (RHF)

uhf

Spin-unrestricted Hartree-Fock (UHF).

ks

Spin-restricted closed and open-shell Kohn-Sham calculations.

uks

Spin-unrestricted Kohn-Sham calculations.

mcscf

MCSCF and CASSCF, including state-averaged calculations

mp2

closed-shell MP2

lmp2

closed-shell local MP2

qcisd(t)

closed-shell quadratic configuration interaction, including perturbative triple-excitation contribution

rs2

Second-order multireference perturbation theory

Various options are available for modifying the convergence thresholds and the optimization method. For details see the reference manual.

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