Abstracts of the CTTC 2019

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Frederik Tielens

Vrije Universiteit Brussel Faculteit Wetenschappen en Bio-ingenieurswetenschappen Pleinlaan 2, B-1050 Brussel, Belgium frederik.tielens@vub.be

Characterization of Self-Assembled Monolayers on Noble Metal Surfaces

Self-assembled monolayers (SAMs) consist of a layer of functionalized long-chain molecules tethered to a solid substrate. SAMs have attracted significant interest of both the fundamental and applied scientific communities. Their presence as a “coating” on a surface is attractive in a number of applications due to the possibility to provide tuning of the surface properties by selectively modifying functional groups on the SAM. Alkanethiols (CH3(CH2)nSH) and alkylthiolate radicals (CH3(CH2)nS∙) adsorption on Au(111) surface is one of the most studied and best-known SAM systems, but also other bioorganic molecules such as amino acids organize at the surface. The nature of the corresponding structure at the surface has been controversial for a long time, as well as other aspects such as the adsorption site on which the thiol chain is anchored, and if the thiol adsorbs by S-H bond breaking process or not. Experimental studies shed some light on both questions, indicating that surface thiol species are attached to Au adatoms, rather than Au atoms in such a bulk-terminated layer, and that it is the movement of these Au-adatom-thiolate moieties that order to produce the SAM structure. Still, some questions remain unsolved such as: At which coverage does this happen, and for which chain length? What is the influence of the presence of defect sites (vacancy and adatoms) on the S-H bond breaking process? The thiols adsorb in a laying down geometry at low coverage, but at which coverage do they straighten up or stand up? In this context we will show here a series of results on the characterization of alkyl thiol SAMs investigated in detail by means of periodic density functional calculations.

References I. Lorenzo Geada, I. Petit, M. Sulpizi and F. Tielens, Surf.Sci. 677 (2018) 271. E. Colombo, G. Belletti, F. Tielens, P. Quaino, Appl. Surf. Sci. 452, (2018) 141. S. Kumar Meena, C. Goldmann, D. Nassoko, M. Seydou, T. Marchandier, S. Moldovan, O. Ersen, F. Ribot, C. Chanéac, C. Sanchez, D. Portehault, F. Tielens, M. Sulpizi, ACS Nano, 11, 7371 (2017). D. Nassoko, M. Seydou, C. Goldmann, C. Chanéac, C. Sanchez, D. Portehault, F. Tielens, Materials Today Chemistry 5, 34, (2017). C. Goldmann, F. Ribot, L.F. Peiretti, P. Quaino, F. Tielens, C. Sanchez, C. Chanéac, D. Portehault, Small, 13, 1604028, (2017) H. Guesmi, N. Luque, E. Santos, F. Tielens, Chem.Eur.J, 23, 1402, (2017). D. Costa, C.-M. Pradier, F. Tielens, L. Savio, Surface Science Reports, 70, 449 (2015).

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Sam Trickey

Dept. of Physics and Quantum Theory Project, Univ. of Florida

Less is More – or – Back to Kohn-Sham

Simplification of widely used meta-generalized-gradient approximation (mGGA) exchange-correlation functionals by removal of their explicit orbital dependence is valuable because it re-incorporates mGGA calculations in the Kohn-Sham framework. Returning to the pure Kohn-Sham local potential framework (rather than the generalized K-S approach almost always used with orbital-dependent mGGA functionals) aids interpretability and improves computational efficiency in large-scale simulations. The talk will summarize how the Laplacian level of refinement can be achieved by use of suitably constructed approximate kinetic energy density functionals (KEDFs). The existence of Laplacian-level deorbitalizations which yield better performance than the original mGGA will be illustrated for molecules with the meta-GGA-made-very-simple functional. Results on standard molecular and condensed-phase test sets obtained from the deorbitalized version of the SCAN functional (“SCAN-L” for SCAN with Laplacian) reproduce the original SCAN error patterns rather well. However, the magnetization of bcc Fe is quite different, an important distinction that will be discussed.

Supported by U.S. Dept. of Energy grants DE-SC 0002139 and DE-SC 0019330.

References D. Mejía-Rodríguez and S.B. Trickey, Phys. Rev. B 98, 115161 (2018); Phys. Rev. A 96, 052512 (2017).

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Luis Rincon

Universidad San Francisco de Quito, Quito, Ecuador

The information content of the Fermi and Coulomb holes

This presentation summarizes two recently proposed information quantities which are employed to visualize the Fermi and Coulomb holes in the real space. The first one is the information content of the Exchange-Correlation hole, calculated from the Kullback–Leibler divergence of the same-spin conditional pair density respect to the marginal probability (χXC). As reported, χXC, can be used to reveal the regions of the space associated to the classical electron pair model [1-3]. The second one is the information content of the correlation hole, which is computed in terms of the Kullback–Leibler divergence of a correlated same-spin conditional pair density respect to the uncorrelated Hartree–Fock pair density (χC) [4-5]. These two information quantities are discussed on the light of the results for high-spin clusters of alkali metals.

1. L. Rincon, R. Almeida, P. L. Contreras and F.J. Torres “The information content of the conditional pair probability” Chem. Phys. Lett. 635, 116 (2015). 2. A.S. Urbina, F.J. Torres and L. Rincon “The electron localization as the information content of the conditional pair probability” J. Chem. Phys. 144, 244104 (2016). 3. L. Rincon, F.J. Torres and R. Almeida “Is the Pauli exclusion the origin of electron localization?” Mol. Phys. 116, 518 (2018) 4. L. Rincon, F.J. Torres, M. Becerra, S. Liu, A. Fritsch and R. Almeida “On the separation of the information content of the Fermi and Coulomb holes and their influence on the electronic properties of molecular systems” Mol. Phys. 117, 610 (2019) 5. F.J. Torres, L. Rincon, C. Zambrano, J.R. Mora and M.A. Mendez “A review on the information content of the pair density as a tool for the description of the electronic properties of molecular systems” Int. J. Quantum Chem. 119, e25763 (2019)

Ángel Martín Pendás

Dpto. Química Física y Analítica. Universidad de Oviedo. Oviedo, Spain

Should charge-shift bonding be reconsidered?

Charge-shift bonding (CSB) was introduced as a distinct third family of electron-pair links that adds to the covalent and ionic tradition. However, the full battery of orbital invariant tools provided by modern real space artillery shows that it is difficult to find CSB signatures outside the original valence-bond framework in which CSB was developed. Here we show that this concept should probably be further investigated.

References J. Luis Casals-Sainz, F. Jiménez-Grávalos, E. Francisco, A. Martín Pendás, Chem. Commun. (2019), DOI: 10.1039/C9CC02123J

Dennis R. Salahub

Department of Chemistry, Centre for Molecular Simulation, Institute for Quantum Science and Technology, Quantum Alberta, University of Calgary, Canada

Towards free-energy profiles for nano-catalyzed chemical reactions in complex environments

I will review our attempts to build somewhat realistic models of nanocatalysis at finite temperature. Current thoughts are to bring in machine-learning techniques to, ideally, define the relevant reaction coordinates/collective variables. Significant progress has been made on such questions in the bio- modeling literature and I would like to understand the new ML methodologies better and to, hopefully, adapt them to the field of nanocatalysis. I am a neophyte, eager for any guidance that CTTC participants might offer, once I have exposed my state of knowledge/ignorance.

Josep M. Luis

University of Girona

Density Functional Theory and Nonlinear Optical Properties

The design of new molecular materials with large nonlinear optical properties (NLOP) remains a challenging task for computational chemistry due to the necessity of accurately computing both electronic and vibrational hyperpolarizabilities of individual molecules as well as the effects of intermolecular interactions. Density Functional Theory (DFT) has proven to be a powerful tool for solving various quantum mechanical problems in a cost-effective way, but DFT performance in the area of NLOP has been under active assessment. We have performed an extensive study of the performance of a diverse set of density functional approximations in predicting the NLOP of hydrogen-bonded complexes using the CCSD(T)/aug-cc-pVTZ level of theory as reference.[1] For all the studied properties, the average absolute errors below 20% can only be obtained using the CAM-B3LYP functional, while LC-BLYP and MN15 are shown to be only slightly less accurate. We reported huge errors in predicting the vibrational second hyperpolarizability by B97X, M06 and M06-2X functionals. This large failure is traced down to a poor determination of third- and fourth-order energy derivatives with respect to normal modes. These results reveal serious flaws of some DFT methods and suggest caution in selecting the appropriate functional to calculate any molecular property that contains vibrational anharmonic contributions.

We have also analyzed the optimal tuning of range-separatiod LC-BLYP functional based on adjusting the attenuating function parameters to satisfy ionization potential theorem. Our results revealed that this approach does not bring any systematic improvement in the predictions of NLOPs. However, we have explored new strategies to tune the range-separation parameter to provide a correct description of NLOP, performing an exhaustive study of the dependency of this parameter in terms of simply quantities.[2] We have found a simple expressions for the optimal value of the attenuating parameter in terms of the second hyperpolarizability values computed at LC-BLYP level that reproduce the CCSD(T) second hyperpolarizabilities. The hyperpolarizabilities obtained with our NLOP-tailored new optimal tuned LC-BLYP are more accurate than the ones obtained with CAM-B3LYP and LC-BLYP functionals. .

[1] Robert Zalesny, Miroslav Medved,Sebastian Sitkiewicz, Eduard Matito, Josep M. Luis, “Can Density Functional Theory Be Trusted for High-Order Electric Properties? The Case of Hydrogen-Bonded Complexes”, J. Chem. Theory Comput., submitted.

[2] Pau Besalú, Sebastian Sitkiewicz, Pedro Salvador, Eduard Matito, Josep M. Luis, in preparation.

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