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| Firstly, molecular orbitals are localised using the intrinsic bonding orbital (IBO) method, described in chapter [[intrinsic basis bonding analysis (IAO/IBO)]]. Occupied orbitals to be embedded are either chosen directly by the user, or the user chooses a set of atoms, and the orbitals sat on these atoms are embedded (see section [[region#selecting the Embedded Orbitals|selecting the Embedded Orbitals]]). The occupied orbitals are reordered putting the embedded orbitals at the top, followed by the other valence orbitals which are placed into the core. The embedded and active/open orbital spaces are then made pseudo-canonical. | Firstly, molecular orbitals are localised using the intrinsic bonding orbital (IBO) method, described in chapter [[intrinsic basis bonding analysis (IAO/IBO)]]. Occupied orbitals to be embedded are either chosen directly by the user, or the user chooses a set of atoms, and the orbitals sat on these atoms are embedded (see section [[region#selecting the Embedded Orbitals|selecting the Embedded Orbitals]]). The occupied orbitals are reordered putting the embedded orbitals at the top, followed by the other valence orbitals which are placed into the core. The embedded and active/open orbital spaces are then made pseudo-canonical. |
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| The virtual orbital space can also be reduced and localised to the embedding region (see section [[region#truncating the virtual space|truncating the virtual space]]). This can only be utilised by the ''FCI'' program within Molpro (and some third-party programs interfaced to Molpro), because the number of molecular orbitals no longer equals the number of basis functions. However, regional local coupled cluster can still be performed by the ''LCC'' program, using its own ''REGION'' directive (see section [[PAO-based local correlation treatments#correlating subsets of electrons (REGION)|correlating subsets of electrons (REGION)]] for details). | The virtual orbital space can also be reduced and localised to the embedding region (see section [[region#truncating the virtual space|truncating the virtual space]]). This can only be utilised by the ''FCI'' program within Molpro (and some third-party programs interfaced to Molpro), because the number of molecular orbitals no longer equals the number of basis functions. However, regional local coupled cluster can be performed [[Local correlation methods with pair natural orbitals (PNOs)|PNO-LCCSD(T)-F12 program]] (recommended, see section [[Local correlation methods with pair natural orbitals (PNOs)#Correlation regions in the PNO program|Correlation regions in the PNO program]]) or by the old (obsolete) [[PAO-based local correlation treatments|DF-LCCSD program]] (see section [[PAO-based local correlation treatments#correlating subsets of electrons (REGION)|correlating subsets of electrons (REGION)]] for details). |
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| ===== Defining the Orbital Spaces ===== | ===== Defining the Orbital Spaces ===== |