Multiconfigurational Quasidegenerate Perturbation Theory Research Articles

Parallel Implementation of the Four-Component Relativistic Quasidegenerate Perturbation Theory with General Multiconfigurational Reference Functions.

A new, efficient parallel algorithm for four-component relativistic generalized multiconfigurational quasidegenerate perturbation theory (GMC-QDPT) introducing Kramers symmetry is implemented. Because it utilizes the independence of the terms in the matrix element computations, this algorithm both speeds up the calculation and reduces the computational resources required for each node. In addition, the amount of memory for two-electron integrals is reduced to three-eigths by Kramers restriction. The algorithm is applied to the d-d excitation energies of the platinum halide complexes, [PtCl4](2-), [PtBr4](2-), and [PtCl6](2-) and to the 6p-7s and 6p-7p excitation energies of the radon atom. It is shown to provide high parallelization efficiency and accurate excitation energies that agree well with experimental data.

Performance of dynamically weighted multiconfiguration self-consistent field and spin-orbit coupling calculations of diatomic molecules of Group 14 elements

The efficacy of several multiconfiguration self-consistent field (MCSCF) methods in the subsequent spin-orbit coupling calculations was studied. Three MCSCF schemes to generate molecular orbitals were analyzed: state-specific, state-averaged, and dynamically weighted MCSCF. With Sn(2)(+) as the representative case, we show that the state-specific MCSCF orbitals lead to discontinuities in potential energy curves when avoided crossings of electronic states occur; this problem can be solved using the state-averaged or dynamically weighted MCSCF orbitals. The latter two schemes are found to give similar results when dynamic electron correlation is considered, which we calculated at the level of multiconfigurational quasidegenerate perturbation theory (MCQDPT). We employed the recently developed Douglas-Kroll spin-orbit adapted model core potential, ZFK3-DK3, and the dynamically weighted MCSCF scheme to calculate the spectroscopic constants of the mono-hydrides and compared them to the results obtained using the older set of potentials, MCP-TZP. We also showed that the MCQDPT tends to underestimate the dissociation energies of the hydrides and discussed to what extent coupled-cluster theory can be used to improve results.

Low-Lying Excited States of C120 and C151: A Multireference Perturbation Theory Study

Excited states of two 7-aminocoumarin derivatives, coumarin 120 (7-amino-4-methylcoumarin) and coumarin 151 (7-amino-4-trifluoromethylcoumarin), were investigated using generalized multiconfigurational quasidegenerate perturbation theory (GMC-QDPT), multiconfigurational quasidegenerate perturbation theory (MC-QDPT) and time-dependent density functional theory (TDDFT) with the B3LYP and CAM-B3LYP functionals. The absorption and fluorescence spectra of C120 and C151 were calculated. We elucidated the characters of the low-lying states of C120 and C151. The absorption spectra calculated with GMC-QDPT and TDDFT B3LYP agreed well with the experimental data, while for the fluorescence spectra, the TDDFT calculations overestimated the fluorescence spectra compared to GMC-QDPT calculations. Utilizing active spaces with large numbers of electrons and orbitals for reference functions, GMC-QDPT showed a better performance than MC-QDPT with a complete active space self-consistent field (CASSCF) reference of active space with smaller number of electrons and orbitals. In our gas phase calculation, we found that the optimized structures for the first excited states have a planar amino group with a CN single bond, while the amino group is pyramidal in the ground state.

An AB Initio Study of the Electronic and Geometric Structure of BIS-of Dipivaloylmethane With Manganese, IRON, and Cobalt

An ab initio study of the structure of Mn(thd)2, Fe(thd)2, and Co(thd)2 complexes in different electronic states is carried out. Quantum chemical calculations are performed using the PC GAMESS program with relativistic effective core pseudopotentials and Gaussian valence triple-zeta basis sets. Calculation methods: DFT/ROB3LYP and CASSCF followed by the inclusion of dynamic electron correlation through multiconfiguration quasi-degenerate second order perturbation theory (MCQDPT2). All three complexes are shown to have a low-spin electronic ground state with a planar structure of the bicyclic fragment at D2h molecular symmetry. The M—O bond is mainly ionic, and M(thd)2 molecules can be considered as an M2+ cation coordinated by two negatively charged bidentate ligands.

Spin-orbit coupling in the ground and low-lying excited states of HF+ and HF-

Electronic structure and spectroscopic properties Be, ωe, ωexe, αe, Te of ground state and the low-lying excited states of HF+ and HF- molecular ions were investigated within scalar relativistic multireference configuration interaction with single and double excitations framework using the GAMESS-US program package. All potential energy curves (PECs) were calculated using the relativistic complete active space self-consistent field/spin-orbit multi-configuration quasi-degenerate perturbation theory (CASSCF/SO-MCQDPT). The curves are all fitted to the analytical potential energy function (APEF), from which accurate spectroscopic constants are derived. The spin-orbit splitting was also been studied, the split value of X\(^{2}{\rm \Pi}\) state of HF+ is determined to be 288.38 cm-1. The calculated properties are in good agreement with the available experimental value. Spectroscopic constants of the ground states of HF- that have never been observed in experiment are obtained. These curves provide an interpretation of the known experimental observations on this system and suggest a number of further experiments which possible provide a critical test of this data.

Comment on “On the Electron Affinity of Nitromethane (CH3NO2)”

Nitrogen dioxide, NO2, being a stable free radical is a reactive gaseous species that is well-known as an anthropogenic pollutant; it is the major initiator of photochemical smog in the troposphere and is an ozone-depleting agent in the stratosphere.(1) Recently, following the successful multireference ab initio determination of an accurate value for the adiabatic electron affinity (AEA) of nitromethane, CH3NO2,(2) it has come to our attention that a similar high-level multiconfigurational convergence of theory and experiment has not been reported for NO2. The previous CH3NO2 study utilizing the N-state multiconfigurational quasi-degenerate perturbation theory of second order method, NS-MCQDPT,(3) showed that for CH3NO2 electron capture is dominated by the -NO2 group, and the level of theory in that study should also apply to gas-phase NO2.

Electronic structure of LaO based on frozen-core four-component relativistic multiconfigurational quasidegenerate perturbation theory

The electronic structure of the LaO molecule is studied using frozen-core four-component multiconfigurational quasidegenerate perturbation theory. The ground state and nine experimentally observed excited states are examined. The ground state is (2)Sigma(1/2)(+) and its gross atomic orbital population is La(5p(5.76)6s(0.83)6p(0.14)p(*(0.21) )d(*(1.17) )f(*(0.26) )) O(2p(4.63)), where p*, d*, and f* are the polarization functions of La that form molecular spinors with O 2ps. We found that it is not necessary to consider the excitation from the O 2p electrons when analyzing the experimental spectra. This validates the foundation of the ligand field theory on diatomic molecules, including the La atom where only one electron is considered. The spectroscopic constants R(e), omega(e), and T(0) calculated for the ground state and low-lying excited states A'((2)Delta(3/2)), A'((2)Delta(5/2)) A((2)Pi(1/2)), and A((2)Pi(3/2)) are in good agreement with the experimental values.

Perspective on Diabatic Models of Chemical Reactivity as Illustrated by the Gas-Phase SN2 Reaction of Acetate Ion with 1,2-Dichloroethane.

Diabatic models are widely employed for studying chemical reactivity in condensed phases and enzymes, but there has been little discussion of the pros and cons of various diabatic representations for this purpose. Here we discuss and contrast six different schemes for computing diabatic potentials for a charge rearrangement reaction. They include (i) the variational diabatic configurations (VDC) constructed by variationally optimizing individual valence bond structures and (ii) the consistent diabatic configurations (CDC) obtained by variationally optimizing the ground-state adiabatic energy, both in the nonorthogonal molecular orbital valence bond (MOVB) method, along with the orthogonalized (iii) VDC-MOVB and (iv) CDC-MOVB models. In addition, we consider (v) the fourfold way (based on diabatic molecular orbitals and configuration uniformity), and (vi) empirical valence bond (EVB) theory. To make the considerations concrete, we calculate diabatic electronic states and diabatic potential energies along the reaction path that connects the reactant and the product ion-molecule complexes of the gas-phase bimolecular nucleophilic substitution (S(N)2) reaction of 1,2-dichloethane (DCE) with acetate ion, which is a model reaction corresponding to the reaction catalyzed by haloalkane dehalogenase. We utilize ab initio block-localized molecular orbital theory to construct the MOVB diabatic states and ab initio multi-configuration quasidegenerate perturbation theory to construct the fourfold-way diabatic states; the latter are calculated at reaction path geometries obtained with the M06-2X density functional. The EVB diabatic states are computed with parameters taken from the literature. The MOVB and fourfold-way adiabatic and diabatic potential energy profiles along the reaction path are in qualitative but not quantitative agreement with each other. In order to validate that these wave-function-based diabatic states are qualitatively correct, we show that the reaction energy and barrier for the adiabatic ground state, obtained with these methods, agree reasonably well with the results of high-level calculations using the composite G3SX and G3SX(MP3) methods and the BMC-CCSD multi-coefficient correlation method. However, a comparison of the EVB gas-phase adiabatic ground-state reaction path with those obtained from MOVB and with the fourfold way reveals that the EVB reaction path geometries show a systematic shift towards the products region, and that the EVB lowest-energy path has a much lower barrier. The free energies of solvation and activation energy in water reported from dynamical calculations based on EVB also imply a low activation barrier in the gas phase. In addition, calculations of the free energy of solvation using the recently proposed SM8 continuum solvation model with CM4M partial atomic charges lead to an activation barrier in reasonable agreement with experiment only when the geometries and the gas-phase barrier are those obtained from electronic structure calculations, i.e., methods i-v. These comparisons show the danger of basing the diabatic states on molecular mechanics without the explicit calculation of electronic wave functions. Furthermore, comparison of schemes i-v with one another shows that significantly different quantitative results can be obtained by using different methods for extracting diabatic states from wave function calculations, and it is important for each user to justify the choice of diabatization method in the context of its intended use.

Investigation of the Ligand-Field States of the Hexaammine Cobalt(III) Ion with Quantum Chemical Methods

The ligand-field (LF) transition energies of the Co(NH3)6(3+) ion have been computed with multiconfiguration quasidegenerate second-order perturbation theory (MCQDPT2). The water solvent was treated with the polarizable continuum model (PCM), and the environment in crystals was modeled by the Co(NH3)6·Cl4(-) complex. The Co-N bond lengths, calculated for the hydrated cation and the Co(NH3)6·Cl4(-) model compound, agree with those in the crystal structures. The vertical transition energies agree with experiment, whereby those based on Co(NH3)6·Cl4(-) are more accurate than those for the hydrated ion. The 0-0 transitions were based on the OPBE geometries of ground and excited (1)T1g, (3)T1g, (5)T2g states of the hydrated ion. The (3)T1g state is the lowest excited state; the (5)T2g state lies higher by >0.6 eV.

Electron affinity of lead: An ab initio four-component relativistic study

The electron affinity of lead is calculated to be 0.403 eV, using four-component relativistic multiconfigurational quasidegenerate perturbation theory. Uncontracted (27 s 23 p 16 d 10 f 2 g) Gaussian-type functions are used to generate valence basis functions (7 s 9 p − 10 p + 8 d ± 10 f ± 2 g ±) in the reduced frozen core approximation, where the KLM shells are frozen. The electrons in the remaining shells are correlated in the perturbation calculations with the (6 s, 6 p) complete active space. The calculated value is within 10% of the experimental value of 0.364 eV. The energy levels of the low-lying excited states of Pb and Pb − are also calculated.

Application of improved virtual orbital based multireference methods to N2, LiF, and C4H6 systems

The improved virtual orbital (IVO) complete active space configuration interaction (CASCI) based multiconfigurational quasidegenerate perturbation theory (MCQDPT) and its single-root version (termed as MRMPPT) are applied to assess the efficacy and the reliability of these two methods. Applications involve the ground and/or excited state potential energy curves (PECs) of N(2), LiF, and C(4)H(6) (butadiene) molecules, systems that are sufficiently complex to assess the applicability of these methods. The ionic-neutral curve crossing involving the lowest two (1)Sigma(+) states of LiF molecule is studied using the IVO-MCQDPT method, while its single-root version (IVO-MRMPPT) is employed to study the ground state PEC for isomerization of butadiene and to model the bond dissociation of N(2) molecule. Comparisons with the standard methods (full CI, coupled cluster with singles and doubles, etc.) demonstrate that the IVO-based MRMPPT and MCQDPT approaches provide smooth and reliable PECs for all the systems studied. The IVO-CASCI method is explored to enable geometry optimization for ground state of C(4)H(6) using numerical energy gradients. The ground spectroscopic constants of N(2) and LiF determined using the numerical gradient based IVO-CASCI method are in accord with experiment and with other correlated calculations. As an illustration, we may point out that the maximum deviation from the experiment in our estimated normal mode frequency of LiF is 34 cm(-1), whereas for the bond length, the maximum error is just 0.012 A.

Perspective on Diabatic Models of Chemical Reactivity as Illustrated by the Gas-Phase S(N)2 Reaction of Acetate Ion with 1,2-Dichloroethane.

Diabatic models are widely employed for studying chemical reactivity in condensed phases and enzymes, but there has been little discussion of the pros and cons of various diabatic representations for this purpose. Here we discuss and contrast six different schemes for computing diabatic potentials for a charge rearrangement reaction. They include (i) the variational diabatic configurations (VDC) constructed by variationally optimizing individual valence bond structures and (ii) the consistent diabatic configurations (CDC) obtained by variationally optimizing the ground-state adiabatic energy, both in the nonorthogonal molecular orbital valence bond (MOVB) method, along with the orthogonalized (iii) VDC-MOVB and (iv) CDC-MOVB models. In addition, we consider (v) the fourfold way (based on diabatic molecular orbitals and configuration uniformity), and (vi) empirical valence bond (EVB) theory. To make the considerations concrete, we calculate diabatic electronic states and diabatic potential energies along the reaction path that connects the reactant and the product ion-molecule complexes of the gas-phase bimolecular nucleophilic substitution (S(N)2) reaction of 1,2-dichloethane (DCE) with acetate ion, which is a model reaction corresponding to the reaction catalyzed by haloalkane dehalogenase. We utilize ab initio block-localized molecular orbital theory to construct the MOVB diabatic states and ab initio multi-configuration quasidegenerate perturbation theory to construct the fourfold-way diabatic states; the latter are calculated at reaction path geometries obtained with the M06-2X density functional. The EVB diabatic states are computed with parameters taken from the literature. The MOVB and fourfold-way adiabatic and diabatic potential energy profiles along the reaction path are in qualitative but not quantitative agreement with each other. In order to validate that these wave-function-based diabatic states are qualitatively correct, we show that the reaction energy and barrier for the adiabatic ground state, obtained with these methods, agree reasonably well with the results of high-level calculations using the composite G3SX and G3SX(MP3) methods and the BMC-CCSD multi-coefficient correlation method. However, a comparison of the EVB gas-phase adiabatic ground-state reaction path with those obtained from MOVB and with the fourfold way reveals that the EVB reaction path geometries show a systematic shift towards the products region, and that the EVB lowest-energy path has a much lower barrier. The free energies of solvation and activation energy in water reported from dynamical calculations based on EVB also imply a low activation barrier in the gas phase. In addition, calculations of the free energy of solvation using the recently proposed SM8 continuum solvation model with CM4M partial atomic charges lead to an activation barrier in reasonable agreement with experiment only when the geometries and the gas-phase barrier are those obtained from electronic structure calculations, i.e., methods i-v. These comparisons show the danger of basing the diabatic states on molecular mechanics without the explicit calculation of electronic wave functions. Furthermore, comparison of schemes i-v with one another shows that significantly different quantitative results can be obtained by using different methods for extracting diabatic states from wave function calculations, and it is important for each user to justify the choice of diabatization method in the context of its intended use.

Spin–orbit coupling effects in the low-lying states of NH+ and NH−

We have carried out relativistic complete active space self-consistent field/spin–orbit multi-configuration quasi-degenerate perturbation theory (CASSCF/SO-MCQDPT) computations on NH+ (X2Π, A2Σ−, B2Δ, C2Σ+) and NH− (X2Π, A2Σ+, B2Π, C2Σ−). The four Λ-S states split into 19 low-lying electronic Ω states of NH+ and 15 low-lying electronic Ω states of NH−, respectively. The potential energy curves (PECs) of these Λ-S states and Ω states of NH+ and NH− are plotted; spectroscopic constants (SCs) of these states are fitted. The splitting constant of NH+ (X2Π) is 36.58 cm−1, which is very close to the experimental value of 34 cm−1. SCs of the excited states (A2Σ+, B2Π, C2Σ−) of NH−, which have never been observed in experiment, are obtained. An excellent agreement is found between the experimentally known values and ours, allowing reliable SC predictions for the Λ-S states and Ω states of NH−.

Modeling of the structure and electronic spectra of green fluorescent protein chromophore

The structure and electronic absorption spectra of a model green fluorescent protein chromophore were studied in the neutral, cationic, and anionic forms in the isolated state. The energies of S0-S1 vertical transitions were calculated using a new method based on a modified multiconfiguration quasi-degenerate perturbation theory (aug-MCQDPT2). This method was used to obtain quantitative estimates of S0-S1 vertical transition energies for chromophores in the isolated state, 2.54, 3.12, and 3.11 eV (the experimental values were 2.59, 3.05, and 2.99 eV) for the anionic, cationic, and neutral forms, respectively.

Potential-energy curves of the CH radical molecule under spin-orbit coupling

Potential energy curves for the ground and excited electronic states of the CH radical molecule were calculated employing spin-orbit multiconfiguration quasi-degenerate perturbation theory (SO-MCQDPT). The results of our present SO-MCQDPT calculation for the CH radical molecule indicate that the ground electronic state of X2Π splits into lower X12Π1/2and higher X22Π3/2Ω states. The excited electronic state of a4Σ–splits into lower a14Σ–1/2and higher a24Σ–3/2, and the excited electronic state of A2Δ splits into lower A12Δ3/2and higher A22Δ5/2. The spin-orbit splittings for the2S+1Λ states X2Π, a4Σ–, and A2Δ are determined to be 25.963, 0.016, and 0.992 cm–1, respectively. The splittings are in good agreement with the experimental data for X2Π and A2Δ , and there are no experimental data for a4Σ–. The potential-energy curves for all calculated bound states of CH are fitted to an analytical potential-energy function in the large range of R = 0.06–0.55 nm, from which accurate spectroscopic parameters are derived. It is the first time that the eight Ω states (X12Π1/2, X22Π3/2, a14Σ–1/2, a24Σ–3/2, A12Δ3/2, A22Δ5/2, B2Σ–1/2, and C2Σ+1/2) generated from the five valence2S+1Λ states (X2Π , a4Σ–, A2Δ, B2Σ–, and C2Σ+) among those dissociating up to H(2Sg)+C(1Dg) have been studied theoretically. In addition, the carbon atom was taken as an example to prove the validity of the SO-MCQDPT method. The agreement between calculated and observed spin-orbit coupling is vert good.PACS Nos.: 31.15.aj, 31.15.xh, 71.70.Ej

Mechanism for the Substitution of an Aqua Ligand of UO2(OH2)5(2+) by Chloride.

Geometry and energy of the reactant (UO2(OH2)5·Cl(+)), the transition state (UO2(OH2)5···Cl(+) (‡)), and the product (UO2Cl(OH2)4·OH2(+)) of the title reaction have been computed with complete active space SCF (geometries and vibrational frequencies) and multiconfiguration quasi-degenerate second-order perturbation theory (total energies). Hydration was treated using the polarizable continuum model. The two investigated active spaces, (12/11) and (12/12), produce the same results. In contrast to the water exchange reaction on UO2(OH2)5(2+), which proceeds via the associative (A) mechanism (which is a two step reaction involving an intermediate with an increased coordination number, UO2(OH2)6(2+)), water substitution by chloride follows the associative interchange (Ia) mechanism (which does not proceed via any intermediate). In this case, structure and imaginary mode of the transition state are not straightforward criteria for the attribution of the substitution mechanism, since they are both typical for the A pathway. The Ia mechanism was derived from the computed intrinsic reaction coordinate, which showed that no intermediate (for example UO2Cl(OH2)5(+)) exists as a local minimum on the potential energy surface. The activation free enthalpy is 31 kJ mol(-1). As for the water exchange reaction, the dissociative mechanism is unlikely to operate because of its higher free activation enthalpy (by ≈25 kJ mol(-1)).

Modeling Photoabsorption of the asFP595 Chromophore

The fluorescent protein asFP595 is a promising photoswitchable biomarker for studying processes in living cells. We present the results of a high level theoretical study of photoabsorption properties of the model asFP595 chromophore molecule in biologically relevant protonation states: anionic, zwitterionic, and neutral. Ground state equilibrium geometry parameters are optimized in the PBE0/(aug)-cc-pVDZ density functional theory approximation. An augmented version of multiconfigurational quasidegenerate perturbation theory (aug-MCQDPT2) following the state-averaged CASSCF/(aug)-cc-pVDZ calculations is used to estimate the vertical S0-S1 excitation energies for all chromophore species. An accuracy of this approach is validated by comparing the computed estimates of the S0-S1 absorption maximum of the closely related chromophore from the DsRed protein to the known experimental value in the gas phase. An influence of the CASSCF active space on the aug-MCQDPT2 excitation energies is analyzed. The zwitterionic form of the asFP595 chromophore is found to be the most sensitive to a particular choice and amount of active orbitals. This observation is explained by the charge-transfer type of the S0-S1 transition involving the entire conjugated pi-electron system for the zwitterionic protonation state. According to the calculation results, the anionic form in the trans conformation is found to possess the most red-shifted absorption band with the maximum located at 543 nm. The bands of the zwitterionic and neutral forms are considerably blue-shifted compared to those of the anionic form. These conclusions are at variance with the results obtained in the TDDFT approximation for the asFP595 chromophore. The absorption wavelengths computed in the aug-MCQDPT2/CASSCF theory are as follows: 543 (535), 470 (476), and 415 (417) nm for the anionic, zwitterionic, and neutral forms of the trans and cis (in parentheses) isomers of the asFP595 chromophore. These data can be used as a reference for further theoretical studies of the asFP595 chromophore in different media and for modeling photoabsorption properties of the asFP595 fluorescent protein.

The electronic structure and energetics of V+-benzene half-sandwiches of different multiplicities: Comparative multireference and single-reference theoretical study

Multireference [complete active space self-consistent field (CASSCF) and multiconfigurational quasidegenerate perturbation theory (MCQDPT)] and single-reference ab initio (Moller-Plesset second order perturbation theory (MP2) and coupled clusters with singles, doubles and noniterative triples [CCSD(T)]) and density functional theory (PBE and B3LYP) electronic structure calculations of V(C(6)H(6))(+) half-sandwich in the states of different multiplicities are described and compared. Detailed analyses of the geometries and electronic structures of the all found states are given; adiabatic and diabatic dissociation energies are estimated. The lowest electronic state of V(C(6)H(6))(+) half-sandwich was found to be the quintet (5)B(2) state with a slightly deformed upside-down-boat-shaped benzene ring and d(4) configuration of V atom, followed by a triplet (3)A(2) state lying about 4 kcal/mol above. The lowest singlet state (1)A(1)(d(4)) lies much ( approximately 28 kcal/mol) higher. MCQDPT calculated adiabatic dissociation energy (53.6 kcal/mol) for the lowest (5)B(2)(d(4)) state agrees well with the current 56.4 (54.4) kcal/mol experimental estimate, giving a preference to the lower one. Compared to MCQDPT, B3LYP hybrid exchange-correlation functional provides the best results, while CCSD(T) performs usually worse. Gradient-corrected PBE calculations tend to systematically overestimate metal-benzene binding in the row quintet<triplet<singlet.

The photophysics of isolated protein chromophores

Gas-phase absorption properties of chromophores of several photoactive proteins have been studied experimentally at the electrostatic heavy-ion storage ring ELISA in Aarhus. The absorption wavelength has been calculated using an augmented effective Hamiltonian technique based on the multiconfigurational quasi-degenerate perturbation theory. The results have been compared to those of widely used state-specific second-order perturbation theory formalisms and their multistate extensions and also to ground-state linear response methods. It would appear that ab initio theory is now at a stage where the intrinsic properties of the chromophore molecules may be predicted with reasonable precision. There is evidence that in terms of absorption there is almost vacuum-like conditions in the hydrophobic interior of some proteins like the green fluorescent protein (GFP). In others, like for example the visual opsins, some significant perturbations are responsible for colour tuning.

Theoretical study of low-lying electronic states of Mn 2 using a newly developed relativistic model core potential

We investigated the electronic structure of low-lying electronic states of Mn 2 using a newly developed relativistic model core potential (spdsMCP). Calculations were performed at complete active space self-consistent field (CASSCF) and second-order multiconfiguration quasidegenerate perturbation theory (MCQDPT2) levels. The MCQDPT2 calculations reveal that the 1Σg + state is the ground state. Calculated spectroscopic constants are very similar to the results of recent all-electron calculations and experimental values, indicating that the spdsMCP works well for Mn 2, which requires a highly correlated calculation. The wave functions of low-lying states are also analyzed at the CASSCF level.