# 13.7.2 Polarization Energy with a Well-defined Basis Set Limit

The definition of polarization energy lowering in the original ALMO-EDA used the full AO space of each fragment as the variational degrees of freedom. This is based on the assumption that the AO basis functions are fragment-ascribable based on their atomic centers. However, this assumption becomes inappropriate when very large basis sets are used, especially those with diffuse functions (e.g. def2-QZVPPD). In such scenarios, basis functions on a given fragment tend to describe other fragments so that the “absolute localization" constraint becomes weaker and finally gets effectively removed. This is why the original ALMO-EDA scheme does not have a well-defined basis set limit for its polarization energy.

To overcome this problem, Horn and Head-Gordon proposed a new definition for the POL term in the ALMO-EDA method based on fragment electrical response functions (FERFs).390 FERFs on a given fragment are prepared by solving CPSCF equations after its SCF solution is found:

 $H_{ai,bj}(\Delta_{\mu})_{bj}=(M_{\mu})_{ai},$ (13.1)

where $\mathbf{H}$ is SCF orbital Hessian and $\mathbf{M}_{\mu}$ is a component ($\mu$) of a multipole matrix with a certain order. The resulting fragment response matrices ($\{\Delta_{\mu}\}$) are a set of $n_{\mathrm{v}}\times n_{\mathrm{o}}$ matrices. Then, a singular value decomposition (SVD) is performed on $\Delta_{\mu}$:

 $(\Delta_{\mu})_{ai}=(L_{\mu})_{ab}(d_{\mu})_{bj}(R_{\mu}^{T})_{ji},$ (13.2)

and the left vectors (not including the null vectors) will be used to construct a truncated virtual space, which is used to define the variational degrees of freedom for the SCFMI problem:

 $\mathbf{V}_{\mu}=\mathbf{C}_{\mathrm{vir}}\mathbf{L}_{\mu},$ (13.3)

where $\mathbf{C}_{\mathrm{vir}}$ denotes the original virtual orbitals of the given fragment.

The basic spirit of using FERFs is to obtain a subset of virtuals that is most pertinent to the electrical polarization of a given fragment, while the redundant variational degrees of freedom (which might be CT-like) are excluded. This scheme is shown to give a well-defined basis set limit for the polarization energy that relies on the SCFMI calculation. The multipole orders (dipole (D), quadrupole (Q), and octopole (O)) included on the RHS of eq. 13.2 decide the span of FERFs on each fragment. Numerical experiments suggest that the inclusion of dipole- and quadrupole-type responses is able to long-range induced electrostatics correctly and also gives a well-defined basis set limit, which is thus recommended as the working basis of the SCFMI problem. The full span of the polarization subspace of fragment $A$ is thus:

 $\mathbf{O}_{A}\oplus\mathrm{span}\{\mathbf{V}_{\mu x},\mathbf{V}_{\mu y},% \mathbf{V}_{\mu z}\}\oplus\mathrm{span}\{\mathbf{V}_{Q2,-2},\mathbf{V}_{Q2,-1}% ,\mathbf{V}_{Q2,0},\mathbf{V}_{Q2,1},\mathbf{V}_{Q2,2}\}.$ (13.4)

Therefore, each occupied orbital will be paired with eight virtual orbitals (if the employed AO basis is large enough).

The polarization subspaces constructed as in eq. 13.4 are non-orthogonal between fragments. Therefore, it is named as the “nDQ" model for polarization. There is another version of this method which enforces interfragment orthogonality between the polarization subspaces and it is correspondingly termed as “oDQ" (or with other multipole orders). The preparation of orthogonal FERFs is more complicated (see ref. 390 for the details) and usually gives less favorable polarization energies. For most general cases, we recommend the use of the “nDQ" model. Calculations using FERFs are performed using the generalized SCFMI procedure introduced in Section 13.7.1.

CHILD_MP
Compute FERFs for fragments and use them as the basis for SCFMI calculations.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
FALSE Do not compute FERFs (use the full AO span of each fragment). TRUE Compute fragment FERFs.
RECOMMENDATION:
Use FERFs to compute polarization energy when large basis sets are used. In an “EDA2" calculation, this $rem variable is set based on the given option automatically. CHILD_MP_ORDERS The multipole orders included in the prepared FERFs. The last digit specifies how many multipoles to compute, and the digits in the front specify the multipole orders: 2: dipole (D); 3: quadrupole (Q); 4: octopole (O). Multipole order 1 is reserved for monopole FERFs which can be used to separate the effect of orbital contraction.559 TYPE: INTEGER DEFAULT: 0 OPTIONS: 21 D 232 DQ 2343 DQO RECOMMENDATION: Use 232 (DQ) when FERF is needed. Example 13.13 Generalized SCFMI calculation for the water dimer using nDQ FERFs. $molecule
0 1
--
0 1
O  -1.551007  -0.114520   0.000000
H  -1.934259   0.762503   0.000000
H  -0.599677   0.040712   0.000000
--
0 1
O   1.350625   0.111469   0.000000
H   1.680398  -0.373741  -0.758561
H   1.680398  -0.373741   0.758561
$end$rem
JOBTYPE           sp
METHOD            wb97x-v
GEN_SCFMAN        true
BASIS             6-31+G(d)
GEN_SCFMAN        true
SCF_ALGORITHM     diis
SCF_CONVERGENCE   8
THRESH            14
SYMMETRY          false
SYM_IGNORE        true
SCF_FINAL_PRINT   1
FRGM_METHOD       stoll
SCFMI_MODE        1 !nonortho gen scfmi
CHILD_MP          true
CHILD_MP_ORDERS   232 !DQ
FD_MAT_VEC_PROD   false
\$end