Active Space Self Research Articles

Electronic Structure Studies with Spin-Orbit Coupling Effect of the Molecule TlI

To study the low–lying electronic states of the TlI molecule, the electronic structure of this molecule has been investigated via an ab initio Complete Active Space Self Consistent Field and the Multireference Configuration Interaction with Davidson correction calculation (CASSCF/MRCI+Q). In the representations of 2s+1Λ(+/−) and (±), the adiabatic potential energy curves (PECs) along with static and transition dipole moment (DM) curves for 19 low-lying electronic states for TlI molecule have been investigated. For the low-lying electronic states of this molecule, the spectroscopic constants Re, Te, ωe, and Be, are provided. Based on the data obtained, this molecule is not a candidate for a Doppler laser cooling study.

The rate constant for the automerization of vinyl radical: A theoretical approach using the Complete Active Space Self Consistent Field method

The Complete Active Space Self Consistent Field method (CASSCF) was utilized to investigate the rate constant for the automerization of vinyl radical. Using an enlarged active space including all valence electrons, thus producing a total number of 213,444 Configuration State Functions (CSFs) the rate constant for the automerization is of the order of 109 s−1 or 1010 s−1 in case of using the vibrational energy splitting tunneling method with the corresponding energy barrier height, involving the zero point vibrational energy corrections, varying between 4.69 and 4.71 kcal/mol (1639.69–1647.34 cm−1). Furthermore depending on the basis set, the ground state vibrational proton tunneling splitting, ΔEsplit, varies between 0.27 and 0.29 cm−1.

Isolation of an Arsenic Diradicaloid with a Cyclic C2 As2 -Core.

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C2As2‐diradicaloid {(IPr)CAs}2 (6) (IPr = C{N(Dipp)CH}2; Dipp = 2,6‐iPr2C6H3). Treatment of (IPr)CH2 (1) with AsCl3 affords the Lewis adduct {(IPr)CH2}AsCl3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl2 (3) and {(IPr)CAsCl}2 (5 a) or [{(IPr)CAs}2Cl]OTf (5 b). Reduction of 5 a (or 5 b) with magnesium turnings gives 6 as a red crystalline solid in 90% yield. Compound 6 featuring a planar C2As2 ring is diamagnetic and exhibits well resolved NMR signals. DFT calculations reveal a singlet ground state for 6 with a small singlet‐triplet energy gap of 8.7 kcal mol−1. The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe)2 and Fe2(CO)9 give rise to {(IPr)CAs(SePh)}2 (7) and {(IPr)CAs}2Fe(CO)4 (8), respectively.

Isolierung eines Arsendiradikaloids mit einem zyklischen C2As2‐Gerüst

AbstractIn diesem Artikel berichten wir über die Synthese, Charakterisierung und Reaktivitätsstudien des ersten zyklischen C2As2‐Diradikaloids {(IPr)CAs}2 (6) (IPr = C{N(Dipp)CH}2; Dipp = 2,6‐iPr2C6H3). Die Umsetzung von (IPr)CH2 (1) mit AsCl3 ergibt das Lewis‐Addukt {(IPr)CH2}AsCl3 (2). Verbindung 2 geht schrittweise eine Dehydrochlorierung ein, die über {(IPr)CH}AsCl2 (3) zu {(IPr)CAsCl}2 (5 a) oder [{(IPr)CAs}2Cl]OTf (5 b) führt. Die Reduktion von 5 a (oder 5 b) mit Magnesiumspänen ergibt 6 als roten kristallinen Feststoff in 90 %iger Ausbeute. Verbindung 6, die einen planaren C2As2‐Ring aufweist, ist diamagnetisch und zeigt gut aufgelöste NMR‐Signale. DFT‐Berechnungen zeigen einen Singulett‐Grundzustand für 6 mit einer kleinen Singulett‐Triplett‐Energielücke von 8.7 kcal mol−1. Der diradikalische Charakter von 6 beträgt 20% (CASSCF, Complete Active Space Self Consistent Field) und 28% (DFT). Die Reaktionen von 6 mit (PhSe)2 bzw. Fe2(CO)9 führen zur Bildung von {(IPr)CAs(SePh)}2 (7) bzw. {(IPr)CAs}2Fe(CO)4 (8).

Wavefunction-based quantum-chemical ab initio calculations for core electron binding energies of small open shell molecules

Core electron binding energies (CEBEs), i.e. ionization energies of 1s core orbitals, are calculated by means of wavefunction-based quantum-chemical ab initio methods for a series of small open-shell molecules containing first-row atoms. The calculations are performed in three steps: (a) Koopmans’ theorem, where the orbitals of the electronic ground state are used unchanged also for the ions, (b) Hartree–Fock or self consistent field (SCF) approximation in which the orbitals are allowed to relax after 1s ionization (ΔSCF), (c) dynamic correlation effects on top of SCF. For open-shell molecules 1s ionization leads to ions in several spin states, mostly to a pair of a triplet and a singlet state. In several cases one or both of these ionic states are only poorly described by a single-reference SCF wavefunction, therefore a multi-reference complete active space self consistent field (CAS-SCF) wavefunction is used instead. The correlation effects are evaluated by means of our multi-reference coupled electron pair approximation program. The accuracy of the calculated CEBEs is in the order of 0.1–0.4 eV. This is in agreement with experimental results for NO and O2. But there exist only very few gas phase data for CEBEs of open-shell molecules.

The Effect of Geometry, Spin, and Orbital Optimization in Achieving Accurate, Correlated Results for Iron-Sulfur Cubanes.

Iron-sulfur clusters comprise an important functional motif in the catalytic centers of biological systems, capable of enabling important chemical transformations at ambient conditions. This remarkable capability derives from a notoriously complex electronic structure that is characterized by a high density of states that is sensitive to geometric changes. The spectral sensitivity to subtle geometric changes has received little attention from correlated, large active space calculations, owing partly to the exceptional computational complexity for treating these large and correlated systems accurately. To provide insight into this aspect, we report the first Complete Active Space Self Consistent Field (CASSCF) calculations for different geometries of the [Fe(II/III)4S4(SMe)4]-2 clusters using two complementary, correlated solvers: spin-pure Adaptive Sampling Configuration Interaction (ASCI) and Density Matrix Renormalization Group (DMRG). We find that the previously established picture of a double-exchange driven magnetic structure, with minute energy gaps (<1 mHa) between consecutive spin states, has a weak dependence on the underlying geometry. However, the spin gap between the singlet and the spin state 2S + 1 = 19, corresponding to a maximal number of Fe-d electrons being unpaired and of parallel spin, is strongly geometry dependent, changing by a factor of 3 upon slight deformations that are still within biologically relevant parameters. The CASSCF orbital optimization procedure, using active spaces as large as 86 electrons in 52 orbitals, was found to reduce this gap compared to typical mean-field orbital approaches. Our results show the need for performing large active space calculations to unveil the challenging electronic structure of these complex catalytic centers and should serve as accurate starting points for fully correlated treatments upon inclusion of dynamical correlation outside the active space.

Theoretical investigation of CH-bond activation by photocatalytic excited SO2 and the effects of C-, N-, S-, and Se-doped TiO2.

The photocatalytic sulfoxidation on TiO2 discovered by Parrino et al. represents a new, interesting and lower energy route for the synthesis of sulfonic acids. Sulfonic acids are important precursors for dyes, detergents and drugs. In the commonly known industrial process, SO2 and a specific hydrocarbon are converted into sulfonic acids using high-energy UV light. In this reaction, SO2 is excited into a metastable triplet state (3SO2), which has the potential to activate a CH-bond of hydrocarbons and start a radical reaction cycle. By introducing TiO2 as a photocatalyst, it has been shown that visible light can be used for the synthesis. This offers the potential to be a cost-effective reaction approach for industrial use. However, experimental studies indicate that the initial excitation mechanism of SO2 on TiO2 is significantly different from the catalyst-free mechanism. Parrino et al. were able to reveal first evidence for the existence of a charge-transfer process from SO2 to the TiO2 surface by means of electrochemical experiments. First theoretical investigations from first principles were able to further substantiate the existence of a charge-transfer. However, to fully understand this mechanism, more accurate methods such as Time Dependent Density Functional Theory (TD-DFT) or ab initio multireference methods such as the Complete Active Space Self Consistent Field (CASSCF) method are required. Furthermore, after understanding the charge-transfer mechanism, the introduction of dopants into TiO2 can be investigated in order to possibly redshift the excitation energy. This might open the route to using lower energy light for the sulfoxidation of hydrocarbons on TiO2 as a new potential industrial reaction for the synthesis of sulfonic acids. In this work, we will study the initial step of the photocatalytic sulfoxidation of hydrocarbons using the TD-DFT and CASSCF methods by using a combined approach consisting of calculations with periodic boundary conditions and a newly constructed embedded cluster model. Furthermore, we will explore the effects of doping by introducing four heteroatoms (C, N, S, and Se) into the TiO2 surfaces anatase[101] and rutile[110] to find a possible enhancement of the photocatalytic reactivity by lowering the electronic excitation energy.

Theoretical electronic structure with rovibrational studies of the molecules YP, YP+ and YP-

Due to the absence of the electronic structure of the YP molecule and its ions in literature, this work is conducted via an ab initio Complete Active Space Self Consistent Field and the Multi-Reference Configuration Interaction with Davidson correction calculation (CASSCF/MRCI + Q) to investigate the low–lying electronic states of these molecules. Adiabatic potential energy curves (PECs) along with static dipole moment (DM) curves for 27, 24, and 21 low-lying electronic states in the representation of 2s+1Λ(+/−) for YP, YP+, and YP- molecules have been investigated, respectively. For the low-lying electronic states of the YP molecule and their anion and cation, the spectroscopic constants Re, Te, ωe, ωexe, Be, De are provided. The rovibrational constants Ev, Bv, Dv, and the abscissa of turning points Rmin and Rmax (up to vibrational level v = 37) are calculated using the canonical functions approach and referring to the calculated data from the PECs. Perturbation theory method is also used to compare our data's validity, as no results are presented in the literature for these molecules.

Quantum HF/DFT-embedding algorithms for electronic structure calculations: Scaling up to complex molecular systems.

In the near future, material and drug design may be aided by quantum computer assisted simulations. These have the potential to target chemical systems intractable by the most powerful classical computers. However, the resources offered by contemporary quantum computers are still limited, restricting the simulations to very simple molecules. In order to rapidly scale up to more interesting molecular systems, we propose the embedding of the quantum electronic structure calculation into a classically computed environment obtained at the Hartree-Fock (HF) or density functional theory (DFT) level of theory. This result is achieved by constructing an effective Hamiltonian that incorporates a mean field potential describing the action of the inactive electrons on a selected Active Space (AS). The ground state of the AS Hamiltonian is then determined by means of the variational quantum eigensolver algorithm. We show that with the proposed HF and DFT embedding schemes, we can obtain significant energy corrections to the reference HF and DFT calculations for a number of simple molecules in their strongly correlated limit (the dissociation regime) as well as for systems of the size of the oxirane molecule.

Theoretical investigation of a novel xylene-based light-driven unidirectional molecular motor.

In this study, the working mechanism of the first light-driven rotary molecular motors used to control an eight-base-pair DNA hairpin has been investigated. In particular, this linker was reported to have promising photophysical properties under physiological conditions, which motivated our work at the quantum mechanical level. Cis-trans isomerization is triggered by photon absorption at wavelengths ranging 300 nm-400 nm, promoting the rotor to the first excited state, and it is mediated by an energy-accessible conical intersection from which the ground state is reached back. The interconversion between the resulting unstable isomer and its stable form occurs at physiological conditions in the ground state and is thermally activated. Here, we compare three theoretical frameworks, generally used in the quantum description of medium-size chemical systems: Linear-Response Time-Dependent Density Functional Theory (LR-TDDFT), Spin-Flip TDDFT (SF-TDDFT), and multistate complete active space second-order perturbation theory on state-averaged complete active space self consistent field wavefunctions (MS-CASPT2//SA-CASSCF). In particular, we show the importance of resorting to a multireference approach to study the rotational cycle of light-driven molecular motors due to the occurrence of geometries described by several configurations. We also assess the accuracy and computational cost of the SF-TDDFT method when compared to MS-CASPT2 and LR-TDDFT.

Laser cooling and electronic structure studies of CaK and its ions CaK±

Aiming at a laser cooling investigation, we have used ab initio complete active space self consistent field (CASSCF)/(MRCI +Q) calculations to study the electronic structure of the diatomic molecule CaK and its molecular ions CaK+ and CaK−. The potential energy curves and the static dipole moment curves have been investigated for the considered molecules along with the spectroscopic constants T e, ω e, B e, and R e, in addition to the values of dipole moment μ e and dissociation energy D e. Overall, 19 and 14 electronic states have been studied respectively for CaK, CaK+, from which 12 for CaK and six for CaK+ have been investigated here for the first time. Our obtained results agree well with data related to states that have been previously examined. Nineteen electronic states have been explored for CaK−, which up to our knowledge have not been previously calculated. The transition dipole moments have been calculated for the lowest Σ+–Σ+ and Σ+–Π transitions along with the Franck–Condon factor, Einstein coefficient, the spontaneous radiative lifetime, and the emission oscillator strength corresponding to the investigated transitions. A ro-vibrational analysis has been done via the canonical function approach, where the vibrational parameters E v, B v, D v, and the turning points R min and R max have been determined. These calculations showed that the molecule CaK is a suitable candidate for Doppler laser cooling, and we propose a laser cooling scheme to this end. The Doppler limit temperature T D and recoil temperature T r have values as low as T D = 51 μK and T r = 156 nK. The results should provide a useful reference for experimental spectroscopic and ultra-cold molecular physics studies.

Electronic structure and mechanism for the uptake of nitric oxide by the Ru(iii) antitumor complex NAMI-A.

Nitric oxide (NO) has well known vasodilation effects in living organisms and its participation in the metastasis of cancer cells through the angiogenesis process has been demonstrated experimentally. Therefore, the uptake of NO has become one focus of investigation to produce anti-metastatic drugs. In this article we have investigated the uptake of NO by the ruthenium based metallodrug trans-tetrachloride(dimethylsulfoxide)imidazole ruthenate(iii) [Im]trans-[RuCl4(Im)(DMSO)], known as New Anti-tumor Metastasis Inhibitor-A (NAMI-A). Electronic structure calculations using Density Functional Theory, DFT, and State-Averaged Complete Active Space Self Consistent Field, SA-CASSCF, with second order perturbation theory corrections, NEVPT2 were carried out to investigate the mechanism involved in the uptake of NO by the Ru-based anticancer metallodrug NAMI-A. The calculations revealed that the reaction takes place at the triplet potential energy surface, with the singlet surface being ∼15 kcal mol−1 shifted to higher energies, and there is a surface crossing to form the most stable singlet product after the reaction takes place at the triplet surface. The spin pairing and electron transfer from the nitric oxide to the metallic fragment takes place at the region of the minimum energy crossing point between the two surfaces. The Ru–NO bond in the {Ru–NO}6 product has ∼10% of the RuIII–NO0 character. The SA-CASSCF/NEVPT2 calculations revealed that the uptake of NO by NAMI-A has a small energy barrier of ∼8 kcal mol−1 and, therefore a rate constant of 11.3 × 106 s−1 at 300 K. In addition, the reaction is thermodynamically favorable, with a Gibbs free energy of ∼30 kcal mol−1. These results show that the uptake of nitric oxide by the NAMI-A complex is kinetically and thermodynamically feasible in biological medium and, therefore, gives support to the anti-angiogenesis theory associated to the mode of action of NAMI-A and other related compounds.

Radiative lifetimes of A2Π, B2Σ+, 22Π, 12Σ–, C2Δ, and a4Σ+ states of carbon monosulfide cation

The transition dipole moments of the dipole–allowed transitions between the X2Σ+, A2Π, B2Σ+, 22Π, 12Σ–, and C2Δ states of the carbon monosulfide cation were calculated using the complete active space self–consistent field method, followed by the internally contracted multireference configuration interaction approach. The transition dipole moments of the spin–forbidden transitions from the a4Σ+ state to the X2Σ+ and A2Π states were computed. The radiative lifetimes were of the order of 1 – 10 µs, 100 ns, 1 µs, 10 – 100 µs, and 1 – 10 µs for the A2Π, B2Σ+, 22Π, 12Σ–, and C2Δ states, respectively. The transition frequencies, Einstein A coefficients, and Franck–Condon factors of all these spontaneous vibronic emissions from these systems were calculated. The emissions from the A2Π – X2Σ+, B2Σ+ – X2Σ+, B2Σ+ – A2Π, 22Π – A2Π, and C2Δ – A2Π systems were strong, suggesting that the 22Π state could be measured in a spectroscopy experiment via 22Π – A2Π transition. Those from the 22Π – B2Σ+, 12Σ– – 22Π, and C2Δ – 22Π systems were weak. The radiative lifetimes of the a4Σ+1/2 and a4Σ+3/2 states were approximately 10 ms. Among these spin–forbidden transitions, the emissions from the a4Σ+3/2 – A2Π3/2 system were relatively strong. The radiative–lifetime distribution of each vibrational level versus the rotational quantum number was evaluated for the A2Π, B2Σ+, 22Π, 12Σ–, C2Δ, and a4Σ+ states.

Ab initio spectroscopy of the aluminum methylene (AlCH2) free radical.

Extensive ab initio investigations of the ground and electronic excited states of the AlCH2 free radical have been carried out in order to predict the spectroscopic properties of this, as yet, undetected species. Difficulties with erratic predictions of the ground state vibrational frequencies, both in the literature and in the present work, have been traced to serious broken-symmetry instabilities in the unrestricted Hartree-Fock orbitals at the ground state equilibrium geometry. The use of restricted open-shell Hartree-Fock or complete active space self consistent field orbitals avoids these problems and leads to consistent and realistic sets of vibrational frequencies for the ground state. Using the internally contracted multireference configuration interaction method with aug-cc-pV(T+d)Z basis sets, we have calculated the geometries, energies, dipole moments, and vibrational frequencies of eight electronic states of AlCH2 and AlCD2. In addition, we have generated Franck-Condon simulations of the expected vibronic structure of the Ã-X̃, B̃-X̃, C̃-X̃, and C̃-Ã band systems, which will be useful in searches for the electronic spectra of the radical. We have also simulated the expected rotational structure of the 0-0 absorption bands of these transitions at modest resolution under supersonic expansion cooled conditions. Our conclusion is that if AlCH2 can be generated in sufficient concentrations in the gas phase, it is most likely detectable through the B̃2A2-X̃2B1 or C̃2A1-X̃2B1 electronic transitions at 515 nm and 372 nm, respectively. Both band systems have vibrational and rotational signatures, even at modest resolution, that are diagnostic of the aluminum methylene free radical.

Theoretical electronic structure with rovibrational calculations of alkali-beryllium molecules BeX (X = K, Rb, Cs)

The low–lying electronic states (doublet and quartet) of the diatomic molecules BeK, BeRb and BeCs have been investigated theoretically based on the ab initio Complete Active Space Self Consistent Field (CASSF), and the Multi-Reference Configuration Interaction with Davidson correction MRCI+ Q. The potential energy (PECs) and dipole moment (DMs) curves have been investigated for the 25, 28, and 24 low-lying electronic states in the representation of 2s+1Λ(+/−) for BeK, BeRb, and BeCs molecules respectively along with the transition dipole moments (TDMs) between the ground state and four excited electronic states, in addition to static dipole polarizability for ground states. The spectroscopic constants, the internuclear distance Re, the rotational constant Be, and the electronic energy Te have been calculated. The vibrational energy Ev, the rotational constant Bv, the centrifugal distortion Dv, and the turning points Rmin and Rmax (up to vibrational level v = 50) were obtained using the canonical function approach. By comparing our data with those given in literature (for several electronic states) shows a good agreement. This work represents investigation of new electronic states for BeX molecules (X = K, Rb and Cs) for the first time.

Active-Space Pair Two-Electron Reduced Density Matrix Theory for Strong Correlation

An active-space variational calculation of the two-electron reduced density matrix (2-RDM) is derived and implemented where the active orbitals are correlated within the pair approximation. The pair approximation considers only doubly occupied configurations of the wave function, which enables the calculation of the 2-RDM at a computational cost of [Formula: see text]. Calculations were performed both with the pair active-space configuration interaction (PASCI) method and the pair active-space self-consistent field (PASSCF) method. The latter includes a mixing of the active and inactive orbitals through unitary transformations. The active-space pair 2-RDM method is applied to the nitrogen molecule, the p-benzyne diradical, a newly synthesized biscobalt complex, and the nitrogenase cofactor FeMoco. The FeMoco molecule is treated in a [120,120] active space. Fractional occupations are recovered in each of these systems, indicating the presence of strong electron correlation.

Radiative properties of the six lowest–lying triplet and four lowest–lying singlet states of boron nitride molecule

This study addresses the transition properties of the X3Π, A3Σ+, B3Σ–, C3Π, D3Π, E3Σ+, a1Σ+, b1Π, c1Δ, and d1Σ+ states of the boron nitride molecule. The potential energy curves and transition dipole moments are calculated using the complete active space self–consistent field method, followed by the valence internally contracted multireference configuration interaction approach. The radiative lifetimes are of the order of 10–8 s for the E3Σ+ state, 10–7–10–8 s for the C3Π state, 10–7 s for the D3Π and d1Σ+ states, 10–6 s for the c1Δ state, 10–5–10–6 s for the B3Σ– state, and 10–5 s for the A3Σ+ and b1Π states. The C3Π – X3Π and E3Σ– – X3Π transitions are strong, followed by the D3Π – X3Π, D3Π – B3Σ–, E3Σ– – A3Σ+, d1Σ+ – a1Σ+, and d1Σ+ – b1Π transitions. The spin–forbidden transitions from the a1Σ+ and b1Π states to the X3Π state are calculated and confirmed to be weak. The radiative lifetimes of all the vibrational levels are approximately 10–1 s for the a1Σ+0+ state. The contribution of the b1Π – X3Π transition to the radiative lifetimes of the b1Π state is sufficiently insignificant to be considered negligible. The Franck–Condon factors, Einstein A coefficients, and band origins of all the spontaneous vibronic emissions from all these transitions involved herein are calculated. The distribution of the radiative lifetime varying with rotational angular quantum number J is investigated at J ≤ 70 for a certain vibrational level υ of the A3Σ+, B3Σ–, C3Π, D3Π, E3Σ+, b1Π, and c1Δ states when υ ≤ 15. The transition properties reported in this study can provide useful guidelines for future investigations, both experimentally and theoretically.

On the Problem of Multiple Minima on the S2 Excited Potential Energy Surface of Benzene: A Restricted Active Space Self Consistent Field Study

AbstractApplication of the Restricted Active Space Self‐Consistent Field Theory on the S2 excited state potential energy surface of benzene shows the existence of three minima. Of these, the first one is planar with a D6h molecular point group, while the other two have a boat type structure with an approximate C2v symmetry. All three minima have a biradicaloid structure and constitute potential candidates for new photochemical pathways on the S2 surface. A common S2/S1 Conical Intersection, which is accessible from all three S2 minima has been found which decays to the S1 potential energy surface (PES) giving benzene S1 minimum as the only photoproduct.

Theoretical electronic structure with rovibrational calculations of the alkali chloride molecules XCl (X = Li, Na, K, Rb, Cs)

The adiabatic potential energy curves and the static dipole moment curves of the low-lying singlet and triplet electronic states in the representation 2s+1Λ(+/−) of the five alkali chloride molecules (LiCl, NaCl, KCl, RbCl and CsCl) have been investigated via ab initio calculations using the state averaged complete active space self consistent field followed by the Multi-reference single and double configuration interaction method with Davidson correction CASSCF/(MRCI + Q). For the molecules under consideration, the spectroscopic constants Te, Re, ωe and Be, the dipole moment μe, and the dissociation energy De have been calculated for the bound states along with the percentage ionic character fionic around the equilibrium position of the ground state. Moreover, the transition dipole moment curves of the X1∑+–(2)1∑+ and X1∑+–(1)1Π transitions have been investigated for the five alkali chloride molecules. A rovibrational calculation was carried out using the canonical functions approach in order to determine the rovibrational constants Ev, Bv and Dv and the abscissas of the turning points Rmin and Rmax for the investigated bound states.

Theoretical electronic structure with rovibrational and dipole moment calculation of the SiS Molecule

The quantitative distribution of the molecular abundances in the universe is a classical problem in astronomy, astrophysics and cosmo-chemistry. Astrophysicists are interested in determining the abundances of molecular species in order to: (1) Know the primordial composition of the solar system, and its relation to the past and present composition of the earth. (2) Have a complete understanding of physical and chemical processes taking place in the stellar atmospheres and the interstellar medium (3) Test the hypotheses that have been proposed for element formation. To investigate the presence of astronomical sources, experimental astrophysicists usually study the wavelength and intensity of light that they emit. Many diatomic molecular species are present in various astrophysical sources, however, the theoretical studies on such species are not enough and information is missing in that area. Knowledge of the electronic structure, Franck-Condon factors (FCFs), and other related quantities for a band system of a diatomic molecule is essential to arrive at its astrophysically significant parameters such as kinetics of the energy transfer, radiative lifetimes, band intensity, vibrational temperature, etc. In this view, the spectroscopic and ro-vibrational constants, FCFs of several electronic states of the diatomic molecule SiS electronic have been evaluated in this work. We performed theoretical calculation of the low-lying electronic state, of the molecule SiS by using the Complete Active Space Self Consistent Field (CASSCF) method followed by the Multi Reference Configuration Interaction with Davidson correction MRCI+Q. The potential energy along with the dipole moment curves of these states have been calculated along with the spectroscopic constants Re, ωe, Be, and Te. Additionally, the Rotation-vibration lines for the considered electronic states were obtained by direct solution of the nuclear motion Schrödinger equation using the canonical approach with program Rovib-1