7 research outputs found
Theoretical Study of the Dissociation Energy of First-Row Metallocenium Ions
The
bond dissociation energy of a series of metallocenium ions,
i.e., the energy difference of the reaction MCp<sub>2</sub><sup>+</sup> â MCp<sup>+</sup> + Cp· (with M = Ti, V, Cr, Mn, Fe,
Co, and Ni), was studied by means of multiconfigurational perturbation
theory (CASPT2, RASPT2, NEVPT2) and restricted coupled cluster theory
(CCSDÂ(T)). From a comparison between the results obtained from these
different methods, and a detailed analysis of their treatment of electron
correlation effects, a set of MCp<sup>+</sup>âCp binding energies
are proposed with an accuracy of 5 kcal/mol. The computed results
are in good agreement with the experimental data measured by threshold
photoelectron photoion coincidence (TPEPICO) spectroscopy but disagree
with the more recent threshold collision-induced dissociation (TCID)
experiments
Cumulant Approximated Second-Order Perturbation Theory Based on the Density Matrix Renormalization Group for Transition Metal Complexes: A Benchmark Study
The complete active
space second order perturbation theory (CASPT2)
can be extended to larger active spaces by using the density matrix
renormalization group (DMRG) as solver. Two variants are commonly
used: the costly DMRG-CASPT2 with exact 4-particle reduced density
matrix (4-RDM) and the cheaper DMRG-cu(4)-CASPT2 in which the 4-cumulant
is discarded. To assess the accuracy and limitations of the latter
variant DMRG-cu(4)-CASPT2 we study the spin state energetics of iron
porphyrin FeÂ(P) and its model compound FeL<sub>2</sub>, a model for
the active center of NiFe hydrogenase, and manganese-oxo porphyrin
MnOÂ(P)<sup>+</sup>; a series of excited states of chromium hexacarbonyl
CrÂ(CO)<sub>6</sub>; and the interconversion of two Cu<sub>2</sub>O<sub>2</sub><sup>2+</sup> isomers. Our results clearly show that PT2 on
top of DMRG is essential in order to obtain quantitative results for
transition metal complexes. Good results were obtained with DMRG-cu(4)-CASPT2
as compared to full CASPT2 and DMRG-CASPT2 in calculations with small-
and medium-sized active spaces. In calculations with large-sized active
spaces (âŒ30 active orbitals), the performance of DMRG-cu(4)-CASPT2
is less impressive due to the errors originating from both the finite
number of renormalized states <i>m</i> and the 4-RDM approximation
Spin State Energetics in First-Row Transition Metal Complexes: Contribution of (3s3p) Correlation and Its Description by Second-Order Perturbation Theory
This
paper presents an in-depth study of the performance of multiconfigurational
second-order perturbation theory (CASPT2, NEVPT2) in describing spin
state energetics in first-row transition metal (TM) systems, including
bare TM ions, TM ions in a field of point charges (TM/PC), and an
extensive series of TM complexes, where the main focus lies on the
(3s3p) correlation contribution to the relative energies of different
spin states. To the best of our knowledge, this is the first systematic
NEVPT2 investigation of TM spin state energetics. CASPT2 has been
employed in several previous studies but was regularly found to be
biased toward high spin states. The bias was attributed to a too low
value of the so-called IPEA shift Ï”, an empirical correction
in the CASPT2 zeroth-order Hamiltonian with a standard value of 0.25
hartree. Based on comparisons with experiment (TM ions) and calculations
with the multireference configuration interaction (TM ions and TM/PC
systems) and coupled-cluster (TM complexes) methods, we demonstrate
in this work that standard CASPT2 works well for valence correlation
and that its bias toward high-spin states is caused by an erratic
description of (3s3p) correlation effects. The latter problem only
occurs for spin transitions involving a ligand field (de)Âexcitation,
not in bare TM ions. At the same time the (3s3p) correlation contribution
also becomes strongly Ï” dependent. The error can be reduced
by increasing Ï” but only at the expense of deteriorating the
CASPT2 description of valence correlation in the TM complexes. The
alternative NEVPT2 method works well for bare TM and TM/PC systems,
but its results for the TM complexes are disappointing, with large
errors both for the valence and (3s3p) correlation contributions to
the relative energies of different spin states
Toward Highly Accurate Spin State Energetics in First-Row Transition Metal Complexes: A Combined CASPT2/CC Approach
In
previous work on the performance of multiconfigurational second-order
perturbation theory (CASPT2) in describing spin state energetics in
first-row transition metal systems [Pierloot et al. J. Chem. Theory Comput. 2017, 13, 537â553], we showed that standard CASPT2 works well for valence
correlation but does not describe the metal semicore (3s3p) correlation
effects accurately. This failure is partially responsible for the
well-known bias toward high-spin states of CASPT2. In this paper,
we expand our previous work and show that this bias could be partly
removed with a combined CASPT2/CC approach: using high-quality CASPT2
with extensive correlation-consistent basis sets for valence correlation
and low-cost CCSDÂ(T) calculations with minimal basis sets for the
metal semicore (3s3p) correlation effects. We demonstrate that this
approach is efficient by studying the spin state energetics of a series
of iron complexes modeling important intermediates in oxidative catalytic
processes in chemistry and biochemistry. On the basis of a comparison
with bare CCSDÂ(T) results from this and previous work, the average
error of the CASPT2/CC approach is estimated at around 2 kcal mol<sup>â1</sup> in favor of high spin states
Atomic Layer Deposition of Ruthenium on Ruthenium Surfaces: A Theoretical Study
Atomic layer deposition (ALD) of
ruthenium using two ruthenium
precursors, i.e., RuÂ(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> (RuCp<sub>2</sub>) and RuÂ(C<sub>5</sub>H<sub>5</sub>)Â(C<sub>4</sub>H<sub>4</sub>N) (RuCpPy), is studied using density functional theory. By investigating
the reaction mechanisms on bare ruthenium surfaces, i.e., (001), (101),
and (100), and H-terminated surfaces, an atomistic insight in the
Ru ALD is provided. The calculated results show that on the Ru surfaces
both RuCp<sub>2</sub> and RuCpPy can undergo dehydrogenation and ligand
dissociation reactions. RuCpPy is more reactive than RuCp<sub>2</sub>. By forming a strong bond between N of Py and Ru of the surface,
RuCpPy can easily chemisorb on the surfaces. The reactions of RuCp<sub>2</sub> on the surfaces are less favorable as the adsorption is not
strong enough. This could be a factor contributing to the higher growth-per-cycle
of Ru using RuCpPy, as observed experimentally. By studying the adsorption
on H-terminated Ru surfaces, we showed that H can prevent the adsorption
of the precursors, thus inhibiting the growth of Ru. Our calculations
indicate that the H content on the surface can have an impact on the
growth-per-cycle. Finally, our simulations also demonstrate large
impacts of the surface structure on the reaction mechanisms. Of the
three surfaces, the (100) surface, which is the less stable and has
a zigzag surface structure, is also the most reactive one
Spin State Energetics and Oxyl Character of Mn-Oxo Porphyrins by Multiconfigurational ab Initio Calculations: Implications on Reactivity
Important electromeric states in
manganese-oxo porphyrins MnOÂ(P)<sup>+</sup> and MnOÂ(PF<sub>4</sub>)<sup>+</sup> (porphyrinato or <i>meso</i>-tetrafluoroporphyrinato)
have been investigated with correlated ab initio methods (CASPT2,
RASPT2), focusing on their possible role in multistate reactivity
patterns in oxygen transfer (OAT) reactions. Due to the lack of oxyl
character, the Mn<sup>V</sup> singlet ground state is kinetically
inert. OAT reactions should therefore rather proceed through thermally
accessible triplet and quintet states that have a more pronounced
oxyl character. Two states have been identified as possible candidates:
a Mn<sup>V</sup> triplet state and a Mn<sup>IV</sup>OÂ(L<sup>âą</sup>a<sub>2<i>u</i></sub>)<sup>+</sup> quintet state. The latter
state is high-lying in MnOÂ(P)<sup>+</sup> but is stabilized by the
substitutions of H by F at the <i>meso</i> carbons (where
the a<sub>2<i>u</i></sub> orbital has a significant amplitude).
Oxyl character and MnâO bond weakening in these two states
stems from the fact that the MnâO Ï* orbitals become
singly (triplet) or doubly occupied (quintet). Moreover, an important
role for the reactivity of the triplet state is also likely to be
played by the Ï bond that has an empty Ï* orbital, because
of the manifest diradical character of this Ï bond, revealed
by the CASSCF wave function. Interestingly, the diradical character
of this bond increases when the MnâO bond is stretched, while
the singly occupied Ï* orbital looses its oxygen radical contribution.
The RASPT2 results were also used as a benchmark for the description
of excited state energetics and MnâO oxyl character with a
wide range of pure and hybrid density functionals. With the latter
functionals both the Mn<sup>V</sup> â Mn<sup>IV</sup> promotion
energy and the diradical character of the Ï bond (with empty
Ï*) are found to be extremely dependent on the contribution
of exact exchange. For this reason, pure functionals are to be preferred
OpenMolcas: From source code to insight
In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already
includes a large number of new developments realized during the transition from
the commercial MOLCAS product to the open-source platform. The paper initially
describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features
of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density
matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations
include an array of additional options and functionalities. The paper proceeds and
describes developments related to explorations of potential energy surfaces. Here
we present methods for the optimization of conical intersections, the simulation of
adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes
features unique to simulations of spectroscopic and magnetic phenomena such as
the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper
describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, a multiscale simulation option
using frozen-density embedding theory and new electronic and muonic basis sets