### Frozen Core Options to Post SCF Methods Keywords

#### FC

This specifies a “frozen core” calculation, and it implies that inner-shells are excluded from the correlation calculation. This is the default calculation mode. Note that FC, Full, RW, and Window are mutually exclusive. It is equivalent to FreezeG2 for the 6-31G and 6-311G basis sets and to FreezeNobleGasCore for all other basis sets, except that the outer s and p core orbitals of 3rd row and later alkali and alkaline earth atoms are not frozen (in accord with the G2/G3/G4 conventions).

#### FreezeNobleGasCore

In post-SCF calculations, the largest noble gas core is frozen. FrzNGC is a synonym for this option.

#### FreezeInnerNobleGasCore

In post-SCF calculations, the next to largest noble gas core is frozen. That is, the outermost core orbitals are retained. FrzINGC and FC1 are synonyms for this option.

#### FreezeG2

Freeze orbitals according to the G2 convention: d orbitals of main group elements are frozen, but the outer sp core of 3rd row and later alkali and alkaline earth elements are kept in the valence.

#### FreezeG3

Freeze orbitals according to the G3 convention.

#### FreezeG4

Freeze orbitals according to the G4 convention.

#### Full

This specifies that all electrons be included in a correlation calculation.

#### RW

The “read window” option means that specific information about which orbitals are retained in the post-SCF calculation will be given in the input file. ReadWindow is a synonym for RW.

The required input section consists of a line specifying the starting and ending orbitals to be retained, followed by a blank line. A value of zero indicates the first or last orbital, depending on where it is used. If the value for the first orbital is negative (-m), then the highest m orbitals are retained; if the value for the last orbital is negative (-n), then the highest n orbitals are frozen. If m is positive and n is omitted, n defaults to 0. If m is negative and n is omitted, then the highest |m| occupied and lowest |m| virtual orbitals are retained.

Here are some examples for a calculation on C_{4}H_{4}:

0,0 |
Equivalent to Full. |

5,0 |
Freezes the 4 core orbitals and keeps all virtual orbitals (equivalent to FC if the basis has a single zeta core). |

5,-4 |
Freezes the four core orbitals and the highest four virtual orbitals. This is the appropriate frozen core for a basis with a double-zeta core. |

6,22 |
Retains orbitals 6 through 22 in the post-SCF phase. For example, since C_{4}H_{4} has 28 electrons, if this is a closed-shell calculation, there will be 14 occupied orbitals, 5 of which will be frozen, so the post-SCF calculation will involve 9 occupied orbitals (orbitals 6–14) and 8 virtual orbitals (orbitals 15–22). |

-6 |
Retains orbitals 9 through 20. |

#### ChkWindow

The window read in during a previous job is recovered from the checkpoint file.

#### ListWindow

Reads a list of orbitals to freeze from the input stream, terminated by a blank line. Two lists are read for unrestricted calculations. A range of orbitals can be specified, e.g.: 2 7–10 14

### The Window Keyword

#### Window=(m[,n])

Window=parameter

This keyword performs the same function as the ReadWindow option, but takes its input as parameters in the route section rather than from the input stream. It is typically used with Output=(Matrix, MO2ElectronIntegrals).

The Window keyword accepts one or two integers as parameters (as with the corresponding option). It also accepts the following parameters, which have the same meanings as the correspondingly-named options: FC, FreezeNobleGasCore, FreezeInnerNobleGasCore, FreezeG2, FreezeG3, FreezeG4, and Full.