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RI-MP2-R12 Calculations

To obtain the R12 correction to the MP2 energy, the data group $rir12 must be added to the control file. A typical run will include the input:


The MP2-R12 ground-state energy is

$\displaystyle E_{\hbox{\tiny MP2-R12}}$ $\displaystyle = E_{\rm MP2} + E_{\rm R12} ,$ (9.24)

where $ E_{\rm MP2}$ is the conventional MP2 energy and $ E_{\rm R12}$ the correction from explicitly-correlated theory. The second term contains contributions from explicitly-correlated geminal basis functions of the form

$\displaystyle \hat Q_{12} f_{12} \vert ij \rangle,$ (9.25)

where $ \vert ij \rangle$ is a two-electron determinant of occupied (semi-)canonical Hartree-Fock spin orbitals, $ f_{12}$ is a correlation factor, which can be either linear $ r_{12}$ or a function of $ r_{12}$, and $ \hat Q_{12}$ defines the doubles excitation space covered by the geminals (it also ensures strong orthogonality to the occupied orbitals). Usually $ \hat Q_{12}$ is chosen to be $ \hat Q_{12} = (1-\hat O_1)(1-\hat O_2) - \hat V_1 \hat V_2$, where $ \hat O_\mu = \sum_{k} \vert\varphi_k(\mu)\rangle\langle\varphi_k(\mu)\vert$ is the projection operator onto the space spanned by the occupied spin orbitals $ \varphi_k$ and $ \hat V_\mu = \sum_{a} \vert\varphi_a(\mu)\rangle\langle\varphi_a(\mu)\vert$ is the projector onto the virtual spin orbitals.

The R12 correction is obtained by minimizing the functional

$\displaystyle F_{\rm R12}$ $\displaystyle = \sum_{i<j} \left\{ {\mathbf c}_{ij}^T {\mathbf B}_{ij} {\mathbf c}_{ij} + 2{\mathbf c}_{ij}^T {\mathbf v}_{ij}\right\}$ (9.26)

with respect to the amplitudes collected in the vector $ {\mathbf c}_{ij}$. The vectors $ {\mathbf v}_{ij}$ and the matrices $ {\mathbf B}_{ij}$ are defined as

$\displaystyle {\mathbf v}_{ij}(kl)$ $\displaystyle = \langle kl\vert f_{12}\hat Q_{12} r_{12}^{-1}\vert ij\rangle ,$ (9.27)
$\displaystyle {\mathbf B}_{ij}(kl,mn)$ $\displaystyle = \langle kl\vert f_{12}\hat Q_{12} (\hat f_1 + \hat f_2 - \varepsilon_i - \varepsilon_j ) \hat Q_{12} f_{12}\vert mn\rangle ,$ (9.28)

in the spin-orbital formalism ($ m,n$ denote spin orbitals and $ \vert mn\rangle$ is a two-electron determinant). $ \hat f_\mu$ is the Fock operator for electron $ \mu$ and $ \varepsilon_k$ is a (semi-)canonical Hartree-Fock orbital energy.

A MP2-R12 calculation is defined through a number of choices concerning the nature of the geminals ($ f_{12}$ and $ \hat Q_{12}$), the geminal excitation space (ijkl or ijij) and approximations in computing the $ B$ matrix (GBC, EBC, $ [\hat T,f_{12}]$). These choices correspond to keywords in the $rir12 data group, explained below.

To run a MP2-R12 calculation, one has to select the auxiliary basis sets cbas, cabs and optionally jkbas. The ricc2 program uses the robust fitting techniques of Ref. [92] for the R12 integrals and the cbas basis is used for both the R12 and the usual MP2 Coulomb integrals. For the density fitting of the Coulomb and exchange matrices of the Fock matrix, the jkbas will be used instead of the cbas basis if it is included in the control file (this is recommended and is achieved using the rijk menu in define). For the RI approximation of the 3- and 4-electron integrals as sums of products of 2-electron integrals, intrinsic to the R12 method, the complementary auxiliary basis (CABS) approach is used [93]. If define is used to set up the cabs basis, the basis sets in the cbas library are used.

The $rir12 data group may be set by choosing the mp2-f12 option in the ricc2 menu when running define. This command activates the mp2-f12 menu, where the default options may be changed if desired:


 ansatz    : CHOOSE ANSATZ                   2      [1,2*,2]
 r12model  : CHOOSE MODEL                    B      [A,A',B]
 comaprox  : COMMUTATOR APPROXIMATION        T      [F+K,T]
 cabs      : CABS ORTHOGONALIZATION          svd    [cho,svd]
 examp     : CHOOSE FORMULATION              fixed  [inv,fixed,noinv]
 pairenergy: PRINT OUT PAIRENERGIES          yes    [yes,no]
 local     : CHOOSE LOCALIZATION METHOD      off    [off,boys,pipek]
 corrfac   : CHOOSE CORRELATION FACTOR       LCG    [R12,LCG]

 * / end    : write $rir12 to file and leave the menu
 &          : go back - leaving $rir12 unchanged...

corresponds to the choice of $ \hat Q_{12}$. Almost all modern MP2-F12 calculations use ansatz 2 (default), which gives much improved energies over ansatz 1 (see Ref. [94] for details). The principal additional cost of using ansatz 2 over ansatz 1 is concerned with the coupling between the R12 and conventional amplitudes. This is avoided by choosing 2*, which corresponds to neglecting EBC (Extended Brillouin Condition) terms in the Fock matrix elements.
is the method of computing the matrices $ {\mathbf B}_{ij}$ (see Ref. [94] for details). The cost and accuracy increases in the progression A, A', B. It is recommended to use B (default). The energies computed using A are then also printed out in the output.
is the method for approximately computing the integrals for the operator $ [\hat T,f_{12}]$, where the matrix representations of F+K or T+V are used. T+V (the core Hamiltonian) is recommended and is the default.
refers to the method of orthogonalising the orbitals in the complementary auxiliary basis. Single-value decomposition (svd) or Choleski decomposition are available. svd is recommended and is the default. The basis set used for CABS is set from the ricc2 menu.
refers to the choice of excitation space. inv is the orbital-invariant merhod of Ref. [95], with amplitudes $ c_{ij}(kl)$. noinv is the original orbital-dependent diagonal "ijij" method of Ref. [95], with amplitudes $ c_{ij}(ij)$ (not recommended, unless in combination with localised orbitals). fixed is the (diagonal and orbital-invariant) rational generator approach of Ref. [96], where the R12 amplitudes are not optimised, but predetermined using the coalescence conditions (default).
controls whether or not the R12 contribution to the MP2 pair energies appear in the output (default yes).
controls which orbitals are used in the calculation. off means that (semi-)canonical Hartree-Fock orbitals are used (default). For calculations using linear-$ r_{12}$ as correlation factor, and r12model A or A', localized orbitals may be used. Both the Boys [97] and Pipek-Mezey [98] methods are available for localisation of the orbitals.
corresponds to the choice of correlation factor $ f_{12}$ in the geminal basis functions. R12 results in a calculation using linear-$ r_{12}$ and LCG results in a calculation using the Slater-type correlation factor with exponent 1.4 $ a_0^{-1}$, represented as a linear combination of six Gaussians (see Ref. [99]).

Further options:

corrfac LCG refers to a further data group for the definition of the correlation factor. When define is used, the default is

  nlcg    6
  slater  1.4000
The nature of the LCG correlation factor may be changed by editing this data group in the control file. For example, to use a Slater-type correlation factor with exponent 1.0 $ a_0^{-1}$, represented as a linear combination of three Gaussians, use
  nlcg    3
  slater  1.0000
Alternatively, the exponents and coefficients of the fit may be given explicitly:
  nlcg    3
  expo1  coef1
  expo2  coef2
  expo3  coef3

For open-shell calculations, two choices of the examp fixed method are available. These are controled by a keyword in the $rir12 data group

  ump2fixed  full      [diag,full]
These differ in the treatment of the $ \alpha\beta$ block, where either only the diagonal excitations enter (with amplitude 0.5) diag, or the equivalent of the spin-adapted singlet and triplet pair excitations enter (as far as possible) full. Note that the diag method with UMP2-F12 yields a different result to that of fixed MP2-F12, even for identical RHF and UHF determinants. However, the diag method is somewhat less expensive than the full method.

The single excitations into the CABS basis are always computed if the CABS Fock matrix elements are anyway required for the R12 calculation (i.e., for ansatz 2, approximation B or comaprox F+K). The computation of CABS singles may be suppressed or enforced by using the cabsingles off/on keyword.

Recommendations for orbital and auxiliary basis sets:

The best orbital basis sets to use for MP2-F12 calculations are probably the cc-pVXZ-F12 basis sets, specially optimised for MP2-F12 calculations. For cabs, we currently recommend to use the cbas aug-cc-pVXZ basis, where X is the cardinal number of the selected orbital basis. The cbas and jkbas basis sets are used for integrals involving both the orbital and cabs basis sets, and we therefore recommend to use the aug-cc-pV(X+1)Z fitting sets. These recommendations err on the side of caution and are likely to be refined as more experience is gained.

next up previous contents index
Next: Parallel RI-MP2 and RI-CC2 Up: Second-Order Approximate Coupled-Cluster (CC2) Previous: Transition Moments   Contents   Index