We previously investigated using high-level ab initio wave functions the electronic
excitation spectra of nitro molecules component or precursor of representative
energetic materials [7, 9–11, 38]. We also studied the photoionization spectra using
the monopole approximation and the SAC-CI wave function N,N-dimethylnitramine,
(CH 3 ) 2 NNO 2 [7], nitramide, H 2 NNO 2 [8] and 1,1-diamino-2,2-dinitroethylene,
known as FOX-7 or DADNE [11]. In this work, in addition to discussing these
works, we present new photoionization results of nitromethane.
2 Methods
We converged the geometries of the N,N-dimethylnitramine, nitramide, H 2 NNO 2
and 1,1-diamino-2,2-dinitroethylene molecules with density functional theory
(DFT) using the B3LYP exchange-correlation functional and respectively the
following Gaussian basis sets: the 6-31+G(d,p) and the aug-cc-pVTZ for the latter
two molecules. The two conformers of nitromethane were converged with the
MP2/6-311 G(3df,3pd) method. Frequency calculations confirmed the minimum
character in each case.
The SAC [33] and the SAC-CI [30–32] wave functions were used respectively
for the ground and excited state calculations of the four molecules. The SAC-CI
wave function is based on a cluster expansion of a CI calculation carried out over
selected configurations. For this wave function, all singly excited configurations are
included without selection, the most important doubly excited configurations are
treated through linked operators while triply and quadruple excited configurations
are generated by products of previous single and double excitations—the so called
unlinked operators. The doubly excited configuration operators for the ground and
excited states are selected according to perturbation threshold coefficients. This
method is called SAC-CI SD (single double) R.
For the calculation of the SAC (ground state) and the SAC-CI (excited states)
wave functions all orbitals were correlated in the four systems. Therefore, the active
space comprised sets of occupied and virtual orbitals for each molecule. All singly
and selected doubly excited orbitals configurations were included in the wave
function. The perturbation selection scheme used to select the important doubly
excited configurations, known as level three scheme, involves two energy thresholds: 1.0 × 10
−6 a.u. for the ground state and 1.0 × 10
−7 a.u. for the excited state.
The SAC wave function was used to obtain the ground state of the neutral molecule. We used the Gaussian 03 program for the geometry optimizations and the
SAC/SAC-CI calculations [17].
For the SAC-CI calculations we used the following basis sets (between
parentheses): N,N-dimethylnitramine (6-31+G(d,p)), nitramide (D95V+(d,p)),
1,1-diamino-2,2-dinitroethylene (6-31+G*) and nitromethane (aug-cc-pVTZ).
The ionization cross sections were computed according to the monopole
approximation [28, 39] which employs the dipole approximation, a plane wave
function for describing the removed electron, the SAC wave function for the ground
Photoionization Spectra and Ionization Potentials …
149
excitation spectra of nitro molecules component or precursor of representative
energetic materials [7, 9–11, 38]. We also studied the photoionization spectra using
the monopole approximation and the SAC-CI wave function N,N-dimethylnitramine,
(CH 3 ) 2 NNO 2 [7], nitramide, H 2 NNO 2 [8] and 1,1-diamino-2,2-dinitroethylene,
known as FOX-7 or DADNE [11]. In this work, in addition to discussing these
works, we present new photoionization results of nitromethane.
2 Methods
We converged the geometries of the N,N-dimethylnitramine, nitramide, H 2 NNO 2
and 1,1-diamino-2,2-dinitroethylene molecules with density functional theory
(DFT) using the B3LYP exchange-correlation functional and respectively the
following Gaussian basis sets: the 6-31+G(d,p) and the aug-cc-pVTZ for the latter
two molecules. The two conformers of nitromethane were converged with the
MP2/6-311 G(3df,3pd) method. Frequency calculations confirmed the minimum
character in each case.
The SAC [33] and the SAC-CI [30–32] wave functions were used respectively
for the ground and excited state calculations of the four molecules. The SAC-CI
wave function is based on a cluster expansion of a CI calculation carried out over
selected configurations. For this wave function, all singly excited configurations are
included without selection, the most important doubly excited configurations are
treated through linked operators while triply and quadruple excited configurations
are generated by products of previous single and double excitations—the so called
unlinked operators. The doubly excited configuration operators for the ground and
excited states are selected according to perturbation threshold coefficients. This
method is called SAC-CI SD (single double) R.
For the calculation of the SAC (ground state) and the SAC-CI (excited states)
wave functions all orbitals were correlated in the four systems. Therefore, the active
space comprised sets of occupied and virtual orbitals for each molecule. All singly
and selected doubly excited orbitals configurations were included in the wave
function. The perturbation selection scheme used to select the important doubly
excited configurations, known as level three scheme, involves two energy thresholds: 1.0 × 10
−6 a.u. for the ground state and 1.0 × 10
−7 a.u. for the excited state.
The SAC wave function was used to obtain the ground state of the neutral molecule. We used the Gaussian 03 program for the geometry optimizations and the
SAC/SAC-CI calculations [17].
For the SAC-CI calculations we used the following basis sets (between
parentheses): N,N-dimethylnitramine (6-31+G(d,p)), nitramide (D95V+(d,p)),
1,1-diamino-2,2-dinitroethylene (6-31+G*) and nitromethane (aug-cc-pVTZ).
The ionization cross sections were computed according to the monopole
approximation [28, 39] which employs the dipole approximation, a plane wave
function for describing the removed electron, the SAC wave function for the ground
Photoionization Spectra and Ionization Potentials …
149