# 4.6.9 occ-RI-K Exchange Algorithm

The occupied orbital RI-K (occ-RI-K) algorithm602 is a new scheme for building the exchange matrix (K) partially in the MO basis using the RI approximation. occ-RI-K typically matches current alternatives in terms of both the accuracy (energetics identical to standard RI-K) and convergence (essentially unchanged relative to conventional methods). On the other hand, this algorithm exhibits significant speedups over conventional integral evaluation (14x) and standard RI-K (3.3x) for a test system, a graphene fragment (C${}_{68}$H${}_{22}$) using cc-pVQZ basis set (4400 basis functions), whereas the speedup increases with the size of the AO basis set. Thus occ-RI-K helps to make larger basis set hybrid DFT calculations more feasible, which is quite desirable for achieving improved accuracy in DFT calculations with modern functionals.

The idea of the occ-RI-K formalism comes from a simple observation that the exchange energy $E_{K}$ and its gradient can be evaluated from the diagonal elements of the exchange matrix in the occupied-occupied block $K_{ii}$, and occupied-virtual block $K_{ia}$, respectively, rather than the full matrix in the AO representation, $K_{\mu\nu}$. Mathematically,

 $\displaystyle E_{K}$ $\displaystyle=$ $\displaystyle-\sum_{\mu\nu}P_{\mu\nu}K_{\mu\nu}$ (4.49) $\displaystyle=$ $\displaystyle-\sum_{\mu\nu}c_{\mu i}K_{\mu\nu}c_{\nu i}$ $\displaystyle=$ $\displaystyle-\sum_{i}K_{ii}$

and

 $\frac{\partial E_{K}}{\partial\Delta_{ai}}=2K_{ai}$ (4.50)

where $\Delta$ is a skew-symmetric matrix used to parameterize the unitary transformation $U$, which represents the variations of the MO coefficients as follows:

 $U=e^{(\Delta-\Delta^{T})}.$ (4.51)

From Eq. 4.49 and 4.50 it is evident that the exchange energy and gradient need just $K_{i\nu}$ rather than $K_{\mu\nu}$.

In regular RI-K one has to compute two quartic terms,965 whereas there are three quartic terms for the occ-RI-K algorithm. The speedup of the latter with respect to former can be explained from the following ratio of operations; refer to Ref. 602 for details.

 $\displaystyle\frac{\mbox{\#~{}of~{}RI-K~{}quartic~{}operations}}{\mbox{\#~{}of% ~{}occ-RI-K~{}quartic~{}operations}}=\frac{oNX^{2}+oN^{2}X}{o^{2}X^{2}+o^{2}NX% +o^{2}NX}=\frac{N(X+N)}{o(X+2N)}$ (4.52)

With a conservative approximation of $X\approx 2N$, the speedup is $\frac{3}{4}(N/o)$. The occ-RI-K algorithm also involves some cubic steps which should be negligible in the very large molecule limit. Tests in the Ref. 602 suggest that occ-RI-K for small systems with large basis will gain less speed than a large system with small basis, because the cubic terms will be more dominant for the former than the latter case.

In the course of SCF iteration, the occ-RI-K method does not require us to construct the exact Fock matrix explicitly. Rather, $k_{i\nu}$ contributes to the Fock matrix in the mixed MO and AO representations ($F_{i\nu}$) and yields orbital gradient and DIIS error vectors for converging SCF. On the other hand, since occ-RI-K does not provide exactly the same unoccupied eigenvalues, the diagonalization updates can differ from the conventional SCF procedure. In Ref. 602, occ-RI-K was found to require, on average, the same number of SCF iterations to converge and to yield accurate energies.

The occ-RI-K can be invoked by either setting OCC_RI_K to be true, or (since Q-Chem 5.2) specifying auxiliary basis set for K using AUX_BASIS_K.

OCC_RI_K
Controls the use of the occ-RI-K approximation for constructing the exchange matrix
TYPE:
LOGICAL
DEFAULT:
False Do not use occ-RI-K.
OPTIONS:
True Use occ-RI-K.
RECOMMENDATION:
Larger the system, better the performance

## 4.6.9.1 occ-RI-K for exchange energy gradient evaluation

A very attractive feature of occ-RI-K framework is that one can compute the exchange energy gradient with respect to nuclear coordinates with the same leading quartic-scaling operations as the energy calculation.

The occ-RI-K formulation yields the following formula for the gradient of exchange energy in global Coulomb-metric RI:

 $\displaystyle E_{K}^{x}$ $\displaystyle=$ $\displaystyle(ij|ij)^{x}$ (4.53) $\displaystyle=$ $\displaystyle\sum_{\mu\nu P}\sum_{ij}c_{\mu i}c_{\nu j}C_{ij}^{P}(\mu\nu|P)^{x% }-\sum_{RS}\sum_{ij}C_{ij}^{R}C_{ij}^{S}(R|S)^{x}.$

The superscript $x$ represents the derivative with respect to a nuclear coordinate. Note that the derivatives of the MO coefficients $c_{\mu i}$ are not included here, because they are already included in the total energy derivative calculation by Q-Chem via the derivative of the overlap matrix.

In Eq. 4.53, the construction of the density fitting coefficients ($C_{\mu\nu}^{P}$) has the worst scaling of $\mathcal{O}(M^{4})$ because it involves MO to AO back transformations:

 $\displaystyle C_{\mu\nu}^{P}=\sum_{ij}c_{\mu i}c_{\nu j}C_{ij}^{P}$ (4.54)

where the operation cost is $o^{2}NX+o[\mathrm{NB2}]X$.

RI_K_GRAD
Turn on the nuclear gradient calculations
TYPE:
LOGICAL
DEFAULT:
FALSE Do not invoke occ-RI-K based gradient
OPTIONS:
TRUE Use occ-RI-K based gradient
RECOMMENDATION:
Use "RI_J false"

Example 4.16  Q-Chem input for a energy and gradient calculations with occ-RI-K method.

$molecule 0 1 C 0.0000000 0.3057430 5.7138876 C 0.0000000 -0.5442831 4.4256275 C 0.0000000 0.3675825 3.1787857 C 0.0000000 -0.4853210 1.8908707 C 0.0000000 0.4264823 0.6439435 C 0.0000000 -0.4264823 -0.6439435 C 0.0000000 0.4853210 -1.8908707 C 0.0000000 -0.3675825 -3.1787857 C 0.0000000 0.5442831 -4.4256275 C 0.0000000 -0.3057430 -5.7138876 H 0.8973039 0.9418895 5.7433715 H -0.8973039 0.9418895 5.7433715 H -0.8965960 -1.1827891 4.4155828 H 0.8965960 -1.1827891 4.4155828 H 0.8966512 1.0057416 3.1940146 H -0.8966512 1.0057416 3.1940146 H -0.8966512 -1.1234851 1.8761493 H 0.8966512 -1.1234851 1.8761493 H 0.8966507 1.0646443 0.6587590 H -0.8966507 1.0646443 0.6587590 H -0.8966507 -1.0646443 -0.6587590 H 0.8966507 -1.0646443 -0.6587590 H 0.8966512 1.1234851 -1.8761493 H -0.8966512 1.1234851 -1.8761493 H -0.8966512 -1.0057416 -3.1940146 H 0.8966512 -1.0057416 -3.1940146 H 0.8965960 1.1827891 -4.4155828 H -0.8965960 1.1827891 -4.4155828 H 0.8973039 -0.9418895 -5.7433715 H 0.0000000 0.3580832 -6.5913113 H -0.8973039 -0.9418895 -5.7433715 H 0.0000000 -0.3580832 6.5913113$end

$rem JOBTYPE force EXCHANGE HF BASIS cc-pVTZ AUX_BASIS cc-pVTZ-JK OCC_RI_K 1 RI_K_GRAD 1 INCFOCK 0 PURECART 1111$end