Description

This method keyword requests a Brueckner Doubles calculation [Dykstra77, Handy89, Kobayashi91]. BD gradients are available [Kobayashi91].

Options

#### T

Requests a Brueckner Doubles calculation with a triples contribution [Handy89] added. BD-T is a synonym for BD(T).

#### TQ

Requests a Brueckner Doubles calculation with triples and quadruples contributions [Raghavachari90] added.

#### FC

All frozen core options are available with this keyword; a frozen core calculation is the default. See the discussion of the FC options for full information.

#### MaxCyc=N

Specifies the maximum number of cycles.

#### Conver=N

Sets the convergence calculations to 10^{-N} on the energy and 10^{-(N-2)} on the wavefunction. The default is N=4 for single points and N=6 for gradients.

#### TWInCore

Whether to store amplitudes and products in memory during higher-order post-SCF calculations. The default is to store these if possible, but to run off disk if memory is insufficient. TWInCore causes the program to terminate if these can not be held in memory, while NoTWInCore prohibits in-memory storage.

#### InCore

Forces the in-memory algorithm. This is very fast when it can be used, but requires N^{4}/4 words of memory. It is normally used in conjunction with SCF=InCore. NoInCore prevents the use of the in-core algorithm.

#### SaveAmplitudes

Saves the converged amplitudes in the checkpoint file for use in a subsequent calculation (e.g., using a larger basis set). Using this option results in a very large checkpoint file, but also may significantly speed up later calculations.

#### ReadAmplitudes

Reads the converged amplitudes from the checkpoint file (if present). Note that the new calculation can use a different basis set, method (if applicable), etc. than the original one.

#### Read

Reads the initial orbitals from the checkpoint file rather than doing an HF calculation. Note that the new calculation can use a different basis set than the original one.

Related Keywords

#### OldFCBD

Requests old-style frozen-core BD (the core orbitals are never changed).

#### NewFCBD

Requests new-style frozen-core BD, in which the core orbitals are updated to conform to the BD condition T1=0, which means the BD Fock matrix is diagonal for these orbitals. This is the default.

Availability

Analytic energies and gradients for BD, numerical gradients for BD(T), and numerical frequencies for all methods. The options FC, T and TQ are not available with analytic gradients. Unrestricted open-shell calculations are available for BD energies and gradients.

Examples

The BD energy appears in the output labeled E(Corr), following the final correlation iteration:

Wavefunction amplitudes converged. E(Corr)= -75.001908213

The energy is given in Hartrees. If triples (or triples and quadruples) were requested, the energy including these corrections appears after the preceding:

```
Time for triples= 0.14 seconds.
T4(BD)= -0.12680940D-03
BD(T)= -0.75002034980D+02 Triples-corrected energy.
```

Last updated on: 05 January 2017. [G16 Rev. A.03]