6.6 Auxiliary Basis (Resolution of the Identity) MP2 Methods

6.6.8 RI-MP2 Method for Complex Basis Functions

Based on the general implementation of complex basis functions in libqints by White, Head-Gordon, and McCurdy,White:2015 an RI-MP2 method for complex resonance energies has been implemented.Hernandez:2019, Hernandez:2020 This method is currently limited to closed-shell cases. The RI approximation can be applied to the complex MP2 energy as well as to the Coulomb and exchange parts of the complex HF energy. The use of the RI approximation is particularly advantageous for electronic resonances since their treatment using complex-scaled methods requires large bases with many diffuse functions. In many cases, RI reduces computation times by a factor of 10 or more. Also, there is no need to include complex-scaled functions in the auxiliary basis set; standard auxiliary bases provide excellent results.Hernandez:2020

The full basis set is supplied through the keyword COMPLEX_BASIS, while BASIS specifies the unscaled part thereof. In complete analogy, the auxiliary basis set is specified using the keywords COMPLEX_AUX_BASIS and AUX_BASIS. The keyword COMPLEX_RI_JK controls whether the RI approximation is invoked only for the MP2 part or for the HF reference as well.

See Section 7.9.7 for more information about electronic resonances, functionalities offered by Q-Chem in this context, and the corresponding keywords.

Example 6.16  Q-Chem input for an RI-MP2 calculation of the complex resonance energy (= Stark-shifted energy and tunnel ionization rate) of N2 in a static electric field of a strength of 0.1 a.u.

$molecule
0 1
N  0.00  0.00  0.55
N  0.00  0.00 -0.55
$end

$rem
jobtype             sp
correlation         RIMP2
basis               6-31G
complex_basis       6-31G*
aux_basis           rimp2-aug-cc-pVDZ
complex_aux_basis   rimp2-aug-cc-pVDZ
complex_ri_jk       true
complex_ccman       true
scf_guess           gwh
scf_convergence     10
complex_exponents   1
complex_theta       80
complex_scf         1
complex_scf_guess   1
complex_n_electrons 0
complex_metscf      1
gen_scfman          true
symmetry            false
sym_ignore          true
thresh              14
purecart            111111
$end

$complex_ccman
stark_z   1000
cs_alpha  1000
cs_theta  0
$end