Q-Chem 4.3 User’s Manual

4.7 Dual-Basis Self-Consistent Field Calculations

The dual-basis approximation [196, 197, 198, 199, 200, 201] to self-consistent field (HF or DFT) energies provides an efficient means for obtaining large basis set effects at vastly less cost than a full SCF calculation in a large basis set. First, a full SCF calculation is performed in a chosen small basis (specified by BASIS2). Second, a single SCF-like step in the larger, target basis (specified, as usual, by BASIS) is used to perturbatively approximate the large basis energy. This correction amounts to a first-order approximation in the change in density matrix, after the single large-basis step:

  \begin{equation}  E_{\mathrm{total}} = E_{\mathrm{small\; basis}} + \mathrm{Tr}[(\Delta \ensuremath{\mathbf{P}})\cdot \textbf{F}]_{\mathrm{large\; basis}} \end{equation}   (4.92)

where $\ensuremath{\mathbf{F}}$ (in the large basis) is built from the converged (small basis) density matrix. Thus, only a single Fock build is required in the large basis set. Currently, HF and DFT energies (SP) as well as analytic first derivatives (FORCE or OPT) are available. [Note: As of version 4.0, first derivatives of unrestricted dual-basis DFT energies—though correct—require a code-efficiency fix. We do not recommend use of these derivatives until this improvement has been made.]

Across the G3 set [202, 203, 204] of 223 molecules, using cc-pVQZ, dual-basis errors for B3LYP are 0.04 kcal/mol (energy) and 0.03 kcal/mol (atomization energy per bond) and are at least an order of magnitude less than using a smaller basis set alone. These errors are obtained at roughly an order of magnitude savings in cost, relative to the full, target-basis calculation.

4.7.1 Dual-Basis MP2

The dual-basis approximation can also be used for the reference energy of a correlated second-order Møller-Plesset (MP2) calculation [197, 201]. When activated, the dual-basis HF energy is first calculated as described above; subsequently, the MO coefficients and orbital energies are used to calculate the correlation energy in the large basis. This technique is particularly effective for RI-MP2 calculations (see Section 5.5), in which the cost of the underlying SCF calculation often dominates.

Furthermore, efficient analytic gradients of the DB-RI-MP2 energy have been developed [199] and added to Q-Chem. These gradients allow for the optimization of molecular structures with RI-MP2 near the basis set limit. Typical computational savings are on the order of 50% (aug-cc-pVDZ) to 71% (aug-cc-pVTZ). Resulting dual-basis errors are only 0.001  in molecular structures and are, again, significantly less than use of a smaller basis set alone.

4.7.2 Basis Set Pairings

We recommend using basis pairings in which the small basis set is a proper subset of the target basis (6-31G into 6-31G*, for example). They not only produce more accurate results; they also lead to more efficient integral screening in both energies and gradients. Subsets for many standard basis sets (including Dunning-style cc-pV$X$Z basis sets and their augmented analogs) have been developed and thoroughly tested for these purposes. A summary of the pairings is provided in Table 4.4; details of these truncations are provided in Figure 4.1.

A new pairing for 6-31G*-type calculations is also available. The 6-4G subset (named r64G in Q-Chem) is a subset by primitive functions and provides a smaller, faster alternative for this basis set regime [200]. A case-dependent switch in the projection code (still OVPROJECTION) properly handles 6-4G. For DB-HF, the calculations proceed as described above. For DB-DFT, empirical scaling factors (see Ref. Steele:2007 for details) are applied to the dual-basis correction. This scaling is handled automatically by the code and prints accordingly.

As of Q-Chem version 3.2, the basis set projection code has also been adapted to properly account for linear dependence [201], which can often be problematic for large, augmented (aug-cc-pVTZ, etc..) basis set calculations. The same standard keyword (LINDEPTHRESH) is utilized for linear dependence in the projection code. Because of the scheme utilized to account for linear dependence, only proper-subset pairings are now allowed.

Like single-basis calculations, user-specified general or mixed basis sets may be employed (see Chapter 7) with dual-basis calculations. The target basis specification occurs in the standard $basis section. The smaller, secondary basis is placed in a similar $basis2 section; the syntax within this section is the same as the syntax for $basis. General and mixed small basis sets are activated by BASIS2=BASIS2_GEN and BASIS2=BASIS2_MIXED, respectively.

BASIS

BASIS2

cc-pVTZ

rcc-pVTZ

cc-pVQZ

rcc-pVQZ

aug-cc-pVDZ

racc-pVDZ

aug-cc-pVTZ

racc-pVTZ

aug-cc-pVQZ

racc-pVQZ

6-31G*

r64G, 6-31G

6-31G**

r64G, 6-31G

6-31++G**

6-31G*

6-311++G(3df,3pd)

6-311G*, 6-311+G*

Table 4.4: Summary and nomenclature of recommended dual-basis pairings
  \[  \begin{array}{cccccc} $H-He:$ &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & &  d \\ &  p \\ s \\ \end{array} &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & &  - \\ &  - \\ s \\ \end{array} &  $Li-Ne:$ &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  d \\ &  p & &  f\\ s & &  d \\ &  p \\ s \\ \end{array} &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  d \\ &  p & &  -\\ s & &  d \\ &  p \\ s \\ \end{array}\\ &  $cc-pVTZ$ &  $rcc-pVTZ$ & &  $cc-pVTZ$ &  $rcc-pVTZ$ \\ \end{array}\\  \]    
  \[  \begin{array}{cccccc} $H-He:$ &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  d \\ &  p & &  f\\ s & &  d \\ &  p \\ s \\ \end{array} &  \scriptsize \begin{array}{cccc} s \\ &  - \\ s & &  - \\ &  p & &  -\\ s & &  - \\ &  - \\ s \\ \end{array} &  $Li-Ne:$ &  \scriptsize \begin{array}{ccccc} s \\ &  p \\ s & &  d \\ &  p & &  f\\ s & &  d & &  g\\ &  p & &  f\\ s & &  d \\ &  p \\ s \\ \end{array} &  \scriptsize \begin{array}{ccccc} s \\ &  p \\ s & &  d \\ &  p & &  -\\ s & &  d & &  -\\ &  p & &  -\\ s & &  d \\ &  p \\ s \\ \end{array}\\ &  $cc-pVQZ$ &  $rcc-pVQZ$ & &  $cc-pVQZ$ &  $rcc-pVQZ$ \\ \end{array}\\  \]    
  \[  \begin{array}{cccccc} $H-He:$ &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & \\ &  p \\ s’ & \\ \end{array} &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & \\ &  - \\ s’ & \\ \end{array} &  $Li-Ne:$ &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & &  d \\ &  p \\ s & &  d’ \\ &  p’ \\ s’ \end{array} &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & &  d \\ &  p \\ s & &  - \\ &  p’ \\ s’ \end{array}\\ &  $aug-cc-pVDZ$ &  $racc-pVDZ$ & &  $aug-cc-pVDZ$ &  $racc-pVDZ$ \\ \end{array} \\  \]    
  \[  \begin{array}{cccccc} $H-He:$ &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & &  d \\ &  p \\ s & &  d’ \\ &  p’ \\ s’ \end{array} &  \scriptsize \begin{array}{ccc} s \\ &  p \\ s & &  - \\ &  p \\ s & &  - \\ &  - \\ s’ \end{array} &  $Li-Ne:$ &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  d \\ &  p & &  f\\ s & &  d \\ &  p & &  f’\\ s & &  d’ \\ &  p’ \\ s’ \end{array} &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  d \\ &  p & &  -\\ s & &  d \\ &  p & &  -\\ s & &  - \\ &  p’ \\ s’ \end{array}\\ &  $aug-cc-pVTZ$ &  $racc-pVTZ$ & &  $aug-cc-pVTZ$ &  $racc-pVTZ$ \\ \end{array} \\  \]    
  \[  \begin{array}{cccccc} $H-He:$ &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  d \\ &  p & &  f\\ s & &  d \\ &  p & &  f’\\ s & &  d’ \\ &  p’ \\ s’ \end{array} &  \scriptsize \begin{array}{cccc} s \\ &  p \\ s & &  - \\ &  p & &  -\\ s & &  - \\ &  p & &  -\\ s & &  - \\ &  - \\ s’ \end{array} &  $Li-Ne:$ &  \scriptsize \begin{array}{ccccc} s \\ &  p \\ s & &  d \\ &  p & &  f\\ s & &  d & &  g\\ &  p & &  f\\ s & &  d & &  g’\\ &  p & &  f’\\ s & &  d’ \\ &  p’ \\ s’ \end{array} &  \scriptsize \begin{array}{ccccc} s \\ &  p \\ s & &  d \\ &  p & &  -\\ s & &  d & &  -\\ &  p & &  -\\ s & &  d & &  -\\ &  p & &  -\\ s & &  - \\ &  p’ \\ s’ \end{array}\\ &  $aug-cc-pVQZ$ &  $racc-pVQZ$ & &  $aug-cc-pVQZ$ &  $racc-pVQZ$ \\ \end{array}  \]    
Figure 4.1: Structure of the truncated basis set pairings for cc-pV(T,Q)Z and aug-cc-pV(D,T,Q)Z. The most compact functions are listed at the top. Primed functions depict $aug$ (diffuse) functions. Dashes indicate eliminated functions, relative to the paired standard basis set. In each case, the truncations for hydrogen and heavy atoms are shown, along with the nomenclature used in Q-Chem.

4.7.3 Job Control

Dual-Basis calculations are controlled with the following $rem. DUAL_BASIS_ENERGY turns on the Dual-Basis approximation. Note that use of BASIS2 without DUAL_BASIS_ENERGY only uses basis set projection to generate the initial guess and does not invoke the Dual-Basis approximation (see Section 4.4.5). OVPROJECTION is used as the default projection mechanism for Dual-Basis calculations; it is not recommended that this be changed. Specification of SCF variables (e.g., THRESH) will apply to calculations in both basis sets.

DUAL_BASIS_ENERGY

Activates dual-basis SCF (HF or DFT) energy correction.


TYPE:

LOGICAL


DEFAULT:

FALSE


OPTIONS:

Analytic first derivative available for HF and DFT (see JOBTYPE)

Can be used in conjunction with MP2 or RI-MP2

See BASIS, BASIS2, BASISPROJTYPE


RECOMMENDATION:

Use Dual-Basis to capture large-basis effects at smaller basis cost. Particularly useful with RI-MP2, in which HF often dominates. Use only proper subsets for small-basis calculation.


4.7.4 Examples

Example 4.67  Input for a Dual-Basis B3LYP single-point calculation.

$molecule
   0 1
   H
   H   1   0.75
$end

$rem
   JOBTYPE             sp
   METHOD              b3lyp
   BASIS               6-311++G(3df,3pd)
   BASIS2              6-311G*
   DUAL_BASIS_ENERGY   true
$end

Example 4.68  Input for a Dual-Basis B3LYP single-point calculation with a minimal 6-4G small basis.

$molecule
   0 1
   H
   H   1   0.75
$end

$rem
   JOBTYPE             sp
   METHOD              b3lyp
   BASIS               6-31G*
   BASIS2              r64G
   DUAL_BASIS_ENERGY   true
$end

Example 4.69  Input for a Dual-Basis RI-MP2 single-point calculation.

$molecule
   0 1
   H
   H   1   0.75
$end

$rem
   JOBTYPE             sp
   METHOD              rimp2
   AUX_BASIS           rimp2-cc-pVQZ
   BASIS               cc-pVQZ
   BASIS2              rcc-pVQZ   
   DUAL_BASIS_ENERGY   true
$end

Example 4.70  Input for a Dual-Basis RI-MP2 geometry optimization.

$molecule
   0 1
   H
   H   1   0.75
$end

$rem
   JOBTYPE             opt
   METHOD              rimp2
   AUX_BASIS           rimp2-aug-cc-pVDZ
   BASIS               aug-cc-pVDZ
   BASIS2              racc-pVDZ
   DUAL_BASIS_ENERGY   true
$end

Example 4.71  Input for a Dual-Basis RI-MP2 single-point calculation with mixed basis sets.

$molecule
   0 1
   H
   O   1   1.1
   H   2   1.1  1  104.5
$end

$rem
   JOBTYPE             opt
   METHOD              rimp2
   AUX_BASIS           aux_mixed
   BASIS               mixed
   BASIS2              basis2_mixed
   DUAL_BASIS_ENERGY   true
$end

$basis
 H 1
 cc-pVTZ
 ****
 O 2
 aug-cc-pVTZ
 ****
 H 3
 cc-pVTZ
 ****
$end

$basis2
 H 1
 rcc-pVTZ
 ****
 O 2
 racc-pVTZ
 ****
 H 3
 rcc-pVTZ
 ****
$end

$aux_basis
 H 1
 rimp2-cc-pVTZ
 ****
 O 2
 rimp2-aug-cc-pVTZ
 ****
 H 3
 rimp2-cc-pVTZ
 ****
$end

4.7.5 Dual-Basis Dynamics

The ability to compute SCF and MP2 energies and forces at reduced cost makes dual-basis calculations attractive for ab initio molecular dynamics simulations. Dual-basis BOMD has demonstrated [205] savings of 58%, even relative to state-of-the-art, Fock-extrapolated BOMD. Savings are further increased to 71% for dual-basis RI-MP2 dynamics. Notably, these timings outperform estimates of extended-Lagrangian (Car-Parrinello) dynamics, without detrimental energy conservation artifacts that are sometimes observed in the latter [206].

Two algorithmic factors make modest but worthwhile improvements to dual-basis dynamics. First, the iterative, small-basis calculation can benefit from Fock matrix extrapolation [206]. Second, extrapolation of the response equations (the so-called “Z-vector” equations) for nuclear forces further increases efficiency [207] . Both sets of keywords are described in Section 9.9, and the code automatically adjusts to extrapolate in the proper basis set when DUAL_BASIS_ENERGY is activated.