CCSD Energies Research Articles

Highly accurate incremental CCSD(T) calculations on aqua- and amine-complexes

In this work, the accuracy of the second-order incremental expansion using the domain-specific basis set approach is tested for 20 cationic metal-aqua and 25 cationic metal-amine complexes. The accuracy of the approach is analysed by the statistical measures range, arithmetic mean, mean absolute deviation, root mean square deviation and standard deviation. Using these measures we find that the error due to the local approximations decreases with increasing basis set. Next we construct a local virtual space using projected atomic orbitals (PAOs). The accuracy of the incremental series in combination with a distance-based truncation of the PAO space is analysed and compared to the convergence of the incremental series within the domain-specific basis set approach. Furthermore, we establish the recently proposed incremental CCSD(T)|MP2 method as a benchmark method to obtain highly accurate CCSD(T) energies. In combination with a basis set of quintuple-ζ quality we establish benchmarks for the binding energies of the investigated complexes. Finally, we use the inc-CCSD(T)|MP2/aV5Z’ binding energies of 45 complexes and 34 dissociation reactions to compute the accuracy of several state of the art density functional theory (DFT) functionals like BP86, B3LYP, CAM-B3LYP, M06, PBE0 and TPSSh. With our implementation of the incremental scheme it was possible to compute the inc-CCSD(T)|MP2/aV5Z’ energy for Al(H2O)3+ 25 (6106 AOs).

Theoretical study on low-lying electronic states of Kr2+, Xe2+, and Rn2+

In this work, the equation-of-motion coupled-cluster approach with spin-orbit coupling (SOC) for ionization potentials (IP) at the singles and doubles level (EOMIP-CCSD) is employed to calculate spectroscopic constants of low-lying states of rare gas dimer ions Kr2(+), Xe2(+), and Rn2(+). Two approaches are proposed to include contributions of triples: (1) energies of these states are calculated by adding the IPs from EOMIP-CCSD and the CCSD(T) energy of the rare gas dimers and (2) CCSD(T) energies without SOC for Rg2(+) are first calculated and energies of these states with SOC are determined subsequently using the SOC matrix between these states. The first approach can provide accurate results for the three most stable states, while overestimates bond lengths for the other states. The second approach has been adopted previously and the SOC matrix element between (2)Σ1∕2 (+) and (2)Π1∕2 states was set to be 1/2 times that of the SOC constant. In our work, the SOC matrix elements are determined from the calculated IPs and reasonable results for these states can be achieved with this approach, which could be useful for experimental works.

A simple non-empirical procedure for spin-component-scaled MP2 methods applied to the calculation of the dissociation energy curve of noncovalently-interacting systems

We present a simple and non-empirical method to determine optimal scaling coefficients, within the (spin-component)-scaled MP2 approach, for calculating intermolecular potential energies of noncovalently-interacting systems. The method is based on an observed proportionality between (spin-component) MP2 and CCSD(T) energies for a wide range of intermolecular distances and allows us to compute with high accuracy a large portion of the dissociation curve at the cost of a single CCSD(T) calculation. The accuracy of the present procedure is assessed for a series of noncovalently-interacting test systems: the obtained results reproduce CCSD(T) quality in all cases and definitely outperform conventional MP2, CCSD and SCS-MP2 results. The difficult case of the beryllium dimer is also considered.

Incremental CCSD(T)(F12)|MP2-F12—A Method to Obtain Highly Accurate CCSD(T) Energies for Large Molecules

In this work, we apply the recently proposed MP2 correction to incremental energies within the domain-specific basis set approach to incrementally expanded CCSD(T)(F12) energies. The approach is tested for a set of 27 molecules with different electronic structures including water clusters, aqua complexes, aliphatic hydrocarbons, alkenes, alkynes, aromatic systems, and amino acids. The root mean squared deviation of the absolute energies with respect to the standard calculation is 1.7 kJ/mol, the mean absolute deviation is 1.2 kJ/mol, and the range is 4.7 kJ/mol. The wall time of the computations is reduced due to the application of a doubly parallel strategy-the independent coupled cluster calculations are performed on up to 70 nodes in parallel, and in addition the computations on one node are performed with the SMP-parallelized coupled cluster code in TURBOMOLE. Using this strategy, we can perform computations in minutes or hours, instead of days or months. Applying the proposed scheme enables us to routinely treat systems with 50 atoms at the CCSD(T)(F12) level in combination with appropriate basis sets to obtain nearly CBS accuracy. Also, larger systems are still feasible on a standard cluster as demonstrated for H3O(+)(H2O)25Cl(-) with 80 atoms.

Thermochemical Parameters and Growth Mechanism of the Boron-Doped Silicon Clusters, SinBq with n = 1–10 and q = −1, 0, +1

A systematic investigation of the boron-doped silicon clusters SinB with n ranging from 1 to 10 in the neutral, anionic, and cationic states is performed using quantum chemical calculations. Lowest-energy minima of the clusters considered are identified on the basis of the B3LYP, G4, and CCSD(T) energies. Total atomization energies and thermochemical properties such as ionization energy, electron affinity, and dissociation energies are obtained using the high accuracy G4 (B3LYP-MP4-CCSD(T)) and CCSD(T)/CBS (complete basis set up to n = 4) methods. Theoretical heats of formation are close to each other and used to assess the available experimental values. The growth mechanism for boron-doped silicon clusters SinB with n = 1–10 emerges as follows: (i) each SinB cluster is formed by adding one excess Si-atom into the smaller sized Sin–1B, rather than by adding B into Sin, (ii) a competition between the exposed (exohedral) and enclosed (endohedral) structures occurs at the size Si8B where both structures beco...

Structures and Stability of Irn(CO)m

The low energy isomers of the Irn(CO)m complexes (n = 1, 2, 3, 4, and 6) were investigated using electronic structure methods at the density functional theory and coupled cluster (CCSD(T) theory levels. The local SVWN5 exchange-correlation functional yielded geometry parameters in good agreement with the available experiment for Irn(CO)m, with calculated bond lengths within 0.01 Å. Ir4(CO)12 is predicted to be the most favored complex for reactions of Irn(CO)m with CO at low temperature, and Ir6(CO)16 is predicted to be formed above room temperature. Smaller Irn(CO)m clusters will nucleate to form Ir4(CO)12 spontaneously. Most of the DFT functionals, especially pure GGA functionals, could not predict consistent reaction energies as compared to CCSD(T). The MP2 and ωB97X-D reaction energies provide upper (more negative) and lower (less negative) estimates of the CCSD(T) values. The average value of the reaction energies at the MP2 and ωB97X-D levels qualitatively match the CCSD(T) energies and the average was used to predict the reaction exothermicity for the Irn(CO)m nucleation reactions where CCSD(T) calculations are not feasible.

Computational thermochemistry of glycolaldehyde

AbstractWe use a variant of the focal point analysis to refine estimates of the relative energies of the four low‐energy torsional conformers of glycolaldehyde. The most stable form is the cis‐cis structure which enjoys a degree of H‐bonding from hydroxyl H to carbonyl O; here dihedral angles τ1 (OCCO) and τ2 (CCOH) both are zero. We optimized structures in both CCSD(T)/aug‐cc‐pVDZ and aug‐cc‐pVTZ; the structures agree within 0.01 Å for bond lengths and 1.0 degrees for valence angles, but the larger basis brings the rotational constants closer to experimental values. According to our extrapolation of CCSD(T) energies evaluated in basis sets ranging to aug‐cc‐pVQZ the trans‐trans form (180°, 180°) has a relative energy of 12.6 kJ/mol. The trans‐gauche conformer (160°, ±75°) is situated at 13.9 kJ/mol and the cis‐trans form (0°, 180°) at 18.9 kJ/mol. Values are corrected for zero point vibrational energy by MP2/aug‐cc‐pVTZ frequencies. Modeling the vibrational spectra is best accomplished by MP2/aug‐cc‐pVTZ with anharmonic corrections. We compute the Watsonian parameters that define the theoretical vibrational‐rotational spectra for the four stable conformers, to assist the search for these species in the interstellar medium. Six transition states are located by G4 and CBS‐QB3 methods as well as extrapolation using energies for structures optimized in CCSD(T)/aug‐cc‐pVDZ structures. We use two isodesmic reactions with two well‐established thermochemical computational schemes G4 and CBS‐QB3 to estimate energy enthalpy and Gibbs energy of formation as well as the entropy of the gas phase system. Our extrapolated electronic energies of species appearing in the isodesmic reactions produce independent values of thermodynamic quantities consistent with G4 and CBS‐QB3. © 2013 Wiley Periodicals, Inc.

Enthalpies of formation of phosphorus and oxygen compounds determined by the correlation consistent composite approach

AbstractThe heats of formation of small molecules (O2, O3, P2, P4, PO, PO2, PO3, P2O, P2O2, and P4O6) have been determined by a modified version of the correlation consistent composite approach (ccCA). The equilibrium geometries and vibrational frequencies were computed via density functional theory, using the M06‐2X exchange‐correlation functional and 6‐31G(2df,p) basis sets. The original methodology requires CCSD(T) energy, which now is obtained by CR‐CCSD(T)_L method. The theoretical heats of formation, whose accuracy is estimated as ranging from ±4 to ±10 kJ mol−1, are closer to the available experimental data, but O3 and P4O6 are exceptions, exceed the desired value of ±10 kJ mol−1. The ccCA result for ozone (159.2 kJ mol−1) shows a deviation in comparison to experimental data (142.67 ± 1.7 kJ mol−1), but it is closer to the calculated value 154.0 kJ mol−1 by the high‐level quantum chemical calculation (W1U; Janoschek and Fabian, J. Mol. Struct. 2006, 780–781, 80). The value of heat formation to P4O6 was estimated to be −1706.3 kJ mol−1. This value is quite different of experimental data, and additional studies are needed to understand this deviation. © 2012 Wiley Periodicals, Inc.

AN EVALUATION OF QUANTUM CHEMICAL CALCULATIONS OF REACTION ENERGIES FOR CATALYTIC ACTIVATION PROCESSES: THE ACTIVATION OF PROPANE BY A RHODIUM CATALYST REVISITED

In this paper we report the state of the art CCSD(T)//MP2 ab initio calculations for the activation of propane by cyclopentadienyl carbonyl rhodium, (Cp)Rh(CO) , using the effective core potential of Hay and Wadt (LANL2DZ) for rhodium atom and the correlated consistent polarized valence double-ξ basis set (cc-pVDZ) for C , H and O atoms. The CCSD(T) energy values are used as reference to assess the effect of electron correlation on the reaction energies, as well as the performance of density functional theory (DFT) energy values using various functionals. An investigation on the accuracy of DFT results is relevant since their use in calculations involving large molecular systems is a computationally efficient strategy that enables us to tackle important problems in organometallics field and supramolecular chemistry. Our results for the small model system show that all DFT functionals used here correctly predict the CCSD(T) energy pattern and also reproduce very satisfactorily the MP2 geometrical parameters. The BP86, PBE1PBE and PW91 functionals exhibited the best agreement with structural parameters and relative energy values as compared with ab initio post-Hartree–Fock results, showing a potential use in theoretical investigations on larger systems.

Quantitative integral cross sections for the H + CO2 → OH + CO reaction from a density functional theory-based potential energy surface

The OH + CO → H + CO(2) reaction is important in combustion, atmospheric, and interstellar chemistry. Whereas the direct reaction has been extensively studied both experimentally and theoretically, the reverse reaction has received relatively less attention. Here we carry out a quasiclassical trajectory study of the hyperthermal H + CO(2)→ OH + CO reaction on a new interpolated potential energy surface based on the M06-2X density functional. The results reveal for the first time quantitative agreement with experiment for the reaction cross sections in the range of relative translational energies 1.2-2.5 eV. We attribute this excellent agreement to both the quality of the M06-2X energies, which closely reproduce CCSD(T) energies, and to the potential surface construction strategy that emphasizes both the direct and reverse reactions.

Electron correlation and the stability of substituted alkenes

Isomerization energies for hexenes (C6H12) were evaluated with ab initio (Hartree–Fock (HF), MP2, SCS-MP2, and CCSD(T)) and several density functional approximation (DFA) methods. CCSD(T)/6-311+G(2d,p) energies were taken as a benchmark standard. The HF method incorrectly predicts that monosubstituted alkenes are more stable than multiply-substituted alkenes. DFAs generally predict the correct stability trends of alkenes (mono-, < di-, < tri-, < tetra-substituted alkenes) but errors in popular functionals, such as B3LYP, can be as large as errors found for alkane hydrocarbon thermochemistries. Some of the HF error is traced back to deficiencies in modeling 1,3-geminal and 1,4-vicinal alkyl–alkyl group interactions, called vinylbranches, and changes in CC and CH bond types (sp3–sp2 CC to sp3–sp3 CC and sp3 CH to sp2 CH). The latter is shown to be more significant. Comparison of CCSD(T) energies of trans-2-butene with 2-methylpropylene and cis-2-butene suggests that geminal vinylbranches are stabilizing while vicinal vinylbranches are destabilizing. B3LYP and other DFAs have much smaller errors than HF theory due to inclusion of correlation energy that better reproduces bond type changes. Copyright © 2011 John Wiley & Sons, Ltd.

Microwave Spectrum, Conformational Composition, and Intramolecular Hydrogen Bonding of (2-Chloroethyl)amine (ClCH2CH2NH2)

The microwave spectrum of (2-chloroethyl)amine, ClCH(2)CH(2)NH(2), has been investigated in the 22-120 GHz region. Five rotameric forms are possible for this compound. In two of these conformers, denoted I and II, the Cl-C-C-N chain of atoms is antiperiplanar, with different orientations of the amino group. The link of the said atoms is synclinal in the three remaining forms, III-V, which differ with respect to the orientation of the amino group. The microwave spectra of four of these conformers, I-IV, have been assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner that rare and weak five-membered N-H···Cl intramolecular hydrogen bonds are formed. The geometries of conformers I and II preclude a stabilization by this interaction. The energy differences between the conformers were obtained from relative intensity measurements of spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum. This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III, 4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311++G(3df,3pd) levels of theory. The CCSD rotational constants and energy differences are in good agreement with their experimental counterparts.

On the stabilization of the sulfate dianion by sulfur dioxide in the gas phase. Theoretical studies on structure and stability of S nO 2n+2 anions, n = 1–4

The stabilization with respect to electron detachment of the sulfate dianion SO 4 2 - in gas phase by addition of SO 2 molecules is being studied. Geometries of SO 4 2 - (SO 2) n and SO 4 1 - (SO) n for n = 1–3 have been optimized using the B3PW91 method. The energies of the smaller clusters have been recalculated at the CCSD and CCSD(T) level based on the B3PW91 geometry. In the gas phase the sulfate monoanion is more stable than the dianion, the latter having an adiabatic electron detachment energy of −1.57 eV. However, with the addition of two SO 2 molecules the dianion becomes more stable by 0.38 eV (CCSD energies). The vertical electron detachment energy, while negative for the lone sulfate dianion (−1.26 eV), turns positive with the addition of a single SO 2 molecule (0.64 eV). Chemically bonded monoanions S 2 O 6 1 - and S 3 O 8 1 - , as well as dianions S 2 O 6 2 - , S 3 O 8 2 - and S 4 O 10 2 - , have been identified.

On the Nature of Stabilization in Weak, Medium, and Strong Charge-Transfer Complexes: CCSD(T)/CBS and SAPT Calculations

Weak, medium, and strong charge-transfer (CT) complexes containing various electron donors (C(2)H(4), C(2)H(2), NH(3), NMe(3), HCN, H(2)O) and acceptors (F(2), Cl(2), BH(3), SO(2)) were investigated at the CCSD(T)/complete basis set (CBS) limit. The nature of the stabilization for these CT complexes was evaluated on the basis of perturbative NBO calculations and DFT-SAPT/CBS calculations. The structure of all of the complexes was determined by the counterpoise-corrected gradient optimization performed at the MP2/cc-pVTZ level, and most of complexes possess a linear-like contact structure. The total stabilization energies lie between 1 and 55 kcal/mol and the strongest complexes contain BH(3) as an electron acceptor. When ordering the electron donors and electron acceptors on the basis of these energies, we obtain the same order as that based on the perturbative E2 charge-transfer energies, which provides evidence that the charge-transfer term is the dominant energy contribution. The CCSD(T) correction term, defined as the difference between the CCSD(T) and MP2 interaction energies, is mostly small, which allows the investigation of the CT complexes of this type at the "cheap" MP2/CBS level. In the case of weak and medium CT complexes (with stabilization energy smaller than about 15 kcal/mol), the dominant stabilization originates in the electrostatic term; the dispersion as well as induction and δ(HF) terms covering the CT energy contribution are, however, important as well. For strong CT complexes, induction energy is the second (after electrostatic) most important energy term. The role of the induction and δ(HF) terms is unique and characteristic for CT complexes. For all CT complexes, the CCSD(T)/CBS and DFT-SAPT/CBS stabilization energies are comparable, and surprisingly, it is true even for very strong CT complexes with stabilization energy close to 50 kcal/mol characteristic by substantial charge transfer (more than 0.3 e). It is thus possible to conclude that perturbative DFT-SAPT analysis is robust enough to be applied even for dative-like complexes with substantial charge transfer.

Parameterization of a B3LYP Specific Correction for Noncovalent Interactions and Basis Set Superposition Error on a Gigantic Data Set of CCSD(T) Quality Noncovalent Interaction Energies

A vast number of non-covalent interaction energies at the counterpoise corrected CCSD(T) level have been collected from the literature to build a diverse new dataset. The whole dataset, which consists of 2027 CCSD(T) energies, includes most of the published data at this level. A large subset of the data was then used to train a novel, B3LYP specific, empirical correction scheme for non-covalent interactions and basis set superposition error (abbreviated as B3LYP-MM). Results obtained with our new correction scheme were directly compared to benchmark results obtained with B3LYP-D3(1) and M06-2X(2) (two popular density functions designed specifically to accurately model non-covalent interactions). For non-covalent complexes dominated by dispersion or dipole-dipole interactions all three tested methods give accurate results with the medium size aug-cc-pVDZ(3-6) basis set with MUE's of 0.27 (B3LYP-MM), 0.32 (B3LYP-D3) and 0.47 kcal/mol (M06-2X) (with explicit counterpoise corrections). These results validate both B3LYP-D3 and M06-2X for interactions of this type using a much larger data set than was presented in prior work. However, our new dispersion correction scheme shows some clear advantages for dispersion and dipole-dipole dominated complexes with the small LACVP* basis set, which is very popular in use due to its low associated computational cost: The MUE for B3LYP-MM with the LACVP* basis set for this subset of complexes (without explicit counterpoise corrections) is only 0.28 kcal/mol, compared to 0.65 kcal/mol for M06-2X or 1.16 kcal/mol for B3LYP-D3. Additionally, our new correction scheme also shows major improvements in accuracy for hydrogen-bonded systems and for systems involving ionic interactions, for example cation-π interactions. Compared to B3LYP-D3 and M06-2X, we also find that our new B3LYP-MM correction scheme gives results of higher or equal accuracy for a large dataset of conformer energies of di- and tripeptides, sugars, and cysteine.

The Jahn−Teller Effect: A Case of Incomplete Theory for d4 Complexes?

We present relativistic calculations at the four-component Dirac-DFT level for the geometries of the series of group 9 monoanionic hexafluorides MF(6)(-), M = Co, Rh, Ir. Highly correlated four-component relativistic CCSD(T) energies were also calculated for the optimized geometries. Spin-orbit coupling effects influence the geometrical preferences for molecular structures: relativistic calculations predict ground states with octahedral symmetries O(h)* for all hexafluorides in this study, while at the nonrelativistic limit, a structural deviation toward D(4h) ground state symmetries is predicted. Our findings suggest that relativistic effects have an important role in molecular structure preferences for the title hexafluorides.

Ab Initio Molecular Dynamics Studies of Tetrasulfur. Dynamics Coupling the C2v Open and D2h Closed Forms of S4

Ab initio molecular dynamics (AIMD) simulations were performed on the closed D(2h) and open C(2v) isomers of tetrasulfur. After a careful calibration of the electronic structure method, the calculations were done using the BPW91/aug-cc-pVTZ method. This combination of method/basis set adequately reproduces the relative benchmark CCSD(T) energy difference [Matus, M.; Dixon, D.; Peterson, K. A.; Harkless, J. A. W.; Francisco, J. S. J. Chem. Phys. 2007, 127, 174305] between these two isomers and, crucially, the fact that the D(2h) structure is a transition state linking two equivalent (mirror images) C(2v) isomers. The trajectories show that the symmetric open C(2v) isomers interconvert when passing through the D(2h) closed transition state structure and that, unlike tetraoxygen, no three-dimensional structures arise. The dynamic vibrational analysis yields peaks in good agreement with the static CCSD(T) harmonic frequencies and explains higher peaks as overtones, thus showing that unlike previous AIMD DFT-based approaches, carefully calibrated exchange-correlation functionals can produce reliable molecular dynamics results for complex PESs as the one corresponding to the lowest singlet of S(4).

Explicitly correlated coupled-cluster theory using cusp conditions. II. Treatment of connected triple excitations

The coupled-cluster singles and doubles method augmented with single Slater-type correlation factors (CCSD-F12) determined by the cusp conditions (also denoted as SP ansatz) yields results close to the basis set limit with only small overhead compared to conventional CCSD. Quantitative calculations on many-electron systems, however, require to include the effect of connected triple excitations at least. In this contribution, the recently proposed [A. Köhn, J. Chem. Phys. 130, 131101 (2009)] extended SP ansatz and its application to the noniterative triples correction CCSD(T) is reviewed. The approach allows to include explicit correlation into connected triple excitations without introducing additional unknown parameters. The explicit expressions are presented and analyzed, and possible simplifications to arrive at a computationally efficient scheme are suggested. Numerical tests based on an implementation obtained by an automated approach are presented. Using a partial wave expansion for the neon atom, we can show that the proposed ansatz indeed leads to the expected (L(max)+1)(-7) convergence of the noniterative triples correction, where L(max) is the maximum angular momentum in the orbital expansion. Further results are reported for a test set of 29 molecules, employing Peterson's F12-optimized basis sets. We find that the customary approach of using the conventional noniterative triples correction on top of a CCSD-F12 calculation leads to significant basis set errors. This, however, is not always directly visible for total CCSD(T) energies due to fortuitous error compensation. The new approach offers a thoroughly explicitly correlated CCSD(T)-F12 method with improved basis set convergence of the triples contributions to both total and relative energies.

On the stabilization of the carbonate dianion by sulfur dioxide

The stabilization in the gas phase of the carbonate dianion [Formula: see text] by SO2 molecules is being investigated. The geometries of various isomers of [Formula: see text] (SO2)n and [Formula: see text] (SO2)n, for n = 1–4, have been optimized by the B3PW91/6−311+G(3df) method. Single-point CCSD and CCSD(T) energies at the DFT-optimized geometries were obtained for n = 1–3, using the 6−311+G(d) basis set. For n = 1 and 2, the monoanionic clusters are adiabatically more stable than the dianionic ones. However, starting at n = 3, they become less stable. The CCSD adiabatic electron detachment energy of the dianionic cluster switches from −0.39 eV for n = 2 to +0.20 eV for n = 3. The vertical electron detachment energy turns positive at n = 2, with a CCSD value of 1.35 eV. Several of the less stable dianionic, and most of the monoionic clusters, are characterized by the transfer of an oxygen atom from CO3 to SO2, forming [Formula: see text] or [Formula: see text] units, owing to [Formula: see text] + CO2 being more stable than [Formula: see text] + SO2. For the stabilization of the sulfate dianion by stepwise hydration, studied both experimentally and theoretically by other groups, a minimum of three water molecules was required.

A Multireference Coupled-Cluster Study of Electronic Excitations in Furan and Pyrrole†

A multireference (MR), general-model-space (GMS), state-universal (SU), coupled-cluster (CC) method that considers singly (S) and doubly (D) excited cluster amplitudes relative to the reference configurations spanning the model space (GMS SU CCSD) is employed to investigate a number of low-lying excited states of two five-membered, six pi -electron aromatic ring molecules, namely, furan and pyrrole. An extended basis set that includes diffuse functions and molecule-centered Rydberg functions is employed. Computed vertical excitation energies are compared with experimental values as well as with other theoretical results, namely, those obtained by the equation-of-motion (EOM) CCSD method or, equivalently, the symmetry adapted cluster configuration interaction (SAC-CI) method, by the MR CI method, by MR perturbation theory, and by other methods whenever available. For excited states dominated by single-excitations the GMS SU CCSD energies represent an improvement relative to the most closely related EOM CCSD results by about 0.1 eV.