Imperial College London

Professor Mike Robb, FRS

Faculty of Natural SciencesDepartment of Chemistry

Chair in Chemistry
 
 
 
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Contact

 

+44 (0)20 7594 5757mike.robb Website

 
 
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Location

 

301cMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
to

383 results found

Tran T, Javier Segarra-Martí J, Bearpark M, Robb MAet al., 2019, Molecular vertical excitation energies studied with first order RASSCF (RAS[1,1]): balancing covalent and ionic excited states, Journal of Physical Chemistry A, ISSN: 1089-5639

RASSCF calculations of vertical excitation energies were carried out on a benchmark set of 19 organic molecules studied by Thiel and co-workers [ J. Chem. Phys. 2008, 128, 134110]. The best results, in comparison with the MS-CASPT2 results of Thiel, were obtained using a RASSCF space that contains at most one hole and one particle in the RAS1 and RAS3 spaces, respectively, which we denote as RAS[1,1]. This subset of configurations recovers mainly the effect of polarization and semi-internal electronic correlation that is only included in CASSCF in an averaged way. Adding all-external correlation by allowing double excitations from RAS1 and RAS2 into RAS3 did not improve the results, and indeed, they were slightly worse. The accuracy of the first-order RASSCF computations is demonstrated to be a function of whether the state of interest can be classified as covalent or ionic in the space of configurations built from orbitals localized onto atomic sites. For covalent states, polarization and semi-internal correlation effects are negligible (RAS[1,1]), while for ionic states, these effects are large (because of inherent diffusiveness of these states compared to the covalent states) and, thus, an acceptable agreement with MS-CASPT2 can be obtained using first-order RASSCF with the extra basis set involving 3p orbitals in most cases. However, for those ionic states that are quasi-degenerate with a Rydberg state or for nonlocal nπ* states, there remains a significant error resulting from all external correlation effects.

Journal article

Robb M, Jenkins AJ, 2019, The Damped Ehrenfest (D-Eh) method: Application to non-adiabatic reaction paths, Computational and Theoretical Chemistry, Vol: 1152, Pages: 53-61, ISSN: 0166-1280

An implementation of the Ehrenfest method with damped velocity is discussed. The method is then applied to study the non-adiabatic reaction paths for two simple chemical systems: the isomerization of the allene radical cation in its excited state and the channel 3 photochemical transformation of benzene to benzvalene. For both systems the initial conditions for the Ehrenfest trajectory were either an adiabatic eigenstate with the geometry close to a conical intersection, or a superposition of eigenstates at the geometry close to a conical intersection. In allene we were able to show that the adiabatic reaction, which passes through a conical intersection, stimulates electron dynamics. In benzene we were able to show the importance of the phase at the conical intersection for the generation of the benzevalene intermediate.

Journal article

Spinlove KE, Richings GW, Robb MA, Worth GAet al., 2018, Curve crossing in a manifold of coupled electronic states: direct quantum dynamics simulations of formamide, FARADAY DISCUSSIONS, Vol: 212, Pages: 191-215, ISSN: 1359-6640

Journal article

Jenkins AJ, Spinlove K, Vacher M, Worth GA, Robb MAet al., 2018, The Ehrenfest method with fully quantum nuclear motion (Qu-Eh): application to charge migration in radical cations, Journal of Chemical Physics, Vol: 149, ISSN: 0021-9606

An algorithm is described for quantum dynamics where an Ehrenfest potential is combined with fully quantum nuclear motion (Quantum-Ehrenfest, Qu-Eh). The method is related to the single-set variational multi-configuration Gaussian approach (vMCG) but has the advantage that only a single quantum chemistry computation is required at each time step since there is only a single time-dependent potential surface. Also shown is the close relationship to the “exact factorization method.” The quantum Ehrenfest method is compared with vMCG for study of electron dynamics in a modified bismethylene-adamantane cation system. Illustrative examples of electron-nuclear dynamics are presented for a distorted allene system and for HCCI+ where one has a degenerate Π system.

Journal article

Polyak I, Bearpark MJ, Robb MA, 2018, Application of the unitary group approach (UGA) to evaluate spindensity for Configuration Interaction (CI) calculations in a basisof S$^{2}$ eigenfunctions, International Journal of Quantum Chemistry, Vol: 118, ISSN: 0020-7608

We present an implementation of the spin-dependent unitary group approachto calculate spin densities for CI calculations in a basis of spinsymmetry-adapted functions. Using S$^{2}$ eigenfunctions helps toreduce the size of configuration space and is beneficial in studiesof the systems where selection of states of specific spin symmetryis crucial. In order to achieve this, we combine the method to calculate$U(n)$ generator matrix elements developed by Robb and Downward~[\onlinecite{downward_1977}]with the approach of Gould and Battle to calculate $U(2n)$ generatormatrix elements~[\onlinecite{battle_1993}]. We also compareand contrast the spin density formulated in terms of the spin-independentunitary generators arising from the group theory formalism and equivalent formulation of the spin density representation in terms of the one- and two-electron charge densities.

Journal article

Polyak I, Jenkins A, Vacher M, Bouduban M, Bearpark M, Robb MAet al., 2018, Charge migration engineered by localisation: electron-nuclear dynamics in polyenes and glycine, Molecular Physics, Vol: 116, Pages: 2474-2489, ISSN: 0026-8976

We demonstrate that charge migration can be ‘engineered’ in arbitrary molecular systems if a single localised orbital – that diabatically follows nuclear displacements – is ionised. Specifically, we describe the use of natural bonding orbitals in Complete Active Space Configuration Interaction (CASCI) calculations to form cationic states with localised charge, providing consistently well-defined initial conditions across a zero point energy vibrational ensemble of molecular geometries. In Ehrenfest dynamics simulations following localised ionisation of -electrons in model polyenes (hexatriene and decapentaene) and -electrons in glycine, oscillatory charge migration can be observed for several femtoseconds before dephasing. Including nuclear motion leads to slower dephasing compared to fixed-geometry electron-only dynamics results. For future work, we discuss the possibility of designing laser pulses that would lead to charge migration that is experimentally observable, based on the proposed diabatic orbital approach.

Journal article

Robb MA, 2018, Theoretical Chemistry for Electronic Excited States, Publisher: Royal Society of Chemistry

Book

Jornet-Somoza J, Deumal M, Borge J, Robb MAet al., 2018, A definition of the magnetic transition temperature using valence bond theory, Journal of Physical Chemistry A, Vol: 122, Pages: 2168-2177, ISSN: 1089-5639

Macroscopic magnetic properties are analyzed using Valence Bond theory. Commonly the critical temperature TC for magnetic systems is associated with a maximum in the energy-based heat capacity Cp(T). Here a more broadly applicable definition of the magnetic transition temperature TC is described using spin moment expectation value (i.e. applying the spin exchange density operator) instead of energy. Namely, the magnetic capacity Cs(T) reflects variation in the spin multiplicity as a function of temperature, which is shown to be related to ∂[χT(T)]/∂T. Magnetic capacity Cs(T) depends on long-range spin interactions that are not relevant in the energy-based heat capacity Cp(T). Differences between Cs(T) and Cp(T) are shown to be due to spin order/disorder within the crystal, that can be monitored via a Valence Bond analysis of the corresponding magnetic wavefunction. Indeed the concept of the Boltzmann spin-alignment order is used to provide information about the spin correlation between magnetic units. As a final illustration, the critical temperature is derived from the magnetic capacity for several molecular magnets presenting different magnetic topolo- gies that have been experimentally studied. A systematic shift between the transition temperatures associated with Cs(T) and Cp(T) is observed. It is demonstrated that this shift can be attributed to the loss of long-range spin correlation. This suggests that the magnetic capacity Cs(T) can be used as a predictive tool for the magnetic topology, and thus for the synthetic chemists.

Journal article

Robb MA, Jenkins AJ, Vacher MA, How Nuclear Motion Affects Coherent Electron Dynamics in Molecules, In Attosecond Molecular Dynamics Ed Marc J. J. Vrakking and Franck Lepine RSC Books

Journal article

Robb MA, Vacher MA, Jenkins A, 2018, How Nuclear Motion Affects Coherent Electron Dynamics in Molecules, Attosecond Molecular Dynamics, Editors: Vrakking, Lepine, Publisher: RSC, Pages: 275-307, ISBN: 978-1-78262-995-5

Knowledge about the electron dynamics in molecules is essential for our understanding of chemical and biological processes. Because of their light mass, electrons are expected to move on the attosecond (1 as = 10− 18 s) timescale. The first synthesis of attosecond pulses in 2001 has opened up the possibility of probing electronic motion on its intrinsic timescale. Excitation or ionisation of a molecule with such a short pulse leads to the coherent population of several electronic states, called an electronic wavepacket. The interference between electronic states in such a superposition, alternating between constructive and destructive, leads to oscillating motion of the electron cloud. This purely quantum process relies on the coherence of the electronic wavepacket. A fundamental challenge is to understand to what extent the electronic wavepacket retains its coherence, i.e., how long the oscillations in the electron cloud survive, in the presence of interactions with the nuclei of the molecule. To address this question, we have developed semi-classical and quantum mechanical methods to simulate the dynamics upon ionisation of polyatomic molecules. The chapter contains a review of the theoretical methods we have developed and some applications illustrating new important physical insights about the predicted decoherence process.

Book chapter

Vacher M, Bearpark M, Robb MA, Malhado JPet al., 2017, Electron dynamics upon ionisation of polyatomic molecules: Coupling to quantum nuclear motion and decoherence, Physical Review Letters, Vol: 118, ISSN: 1079-7114

Knowledge about the electronic motion in molecules is essential for our understanding of chemicalreactions and biological processes. The advent of attosecond techniques opens up the possibility toinduce electronic motion, observe it in real time and potentially steer it. A fundamental questionremains the factors influencing electronic decoherence and the role played by nuclear motion in thisprocess. Here, we simulate the dynamics upon ionisation of the polyatomic molecules para-xyleneand modified bismethylene-adamantane, with a quantum mechanical treatment of both electron andnuclear dynamics using the direct dynamics variational multi-configuration Gaussian method. Oursimulations give new important physical insights about the expected decoherence process. We haveshown that the decoherence of electron dynamics happens on the time scale of a few femtoseconds,with the interplay of different mechanisms: thedephasingis responsible for the fast decoherencewhile thenuclear overlap decaymay actually help maintaining it and is responsible for small revivals.

Journal article

Spinlove KE, Vacher M, Bearpark M, Robb MA, Worth GAet al., 2017, Using quantum dynamics simulations to follow the competition between charge migration and charge transfer in polyatomic molecules, CHEMICAL PHYSICS, Vol: 482, Pages: 52-63, ISSN: 0301-0104

Journal article

Orr-Ewing AJ, Verlet JRR, Penfold TJ, Minns RS, Minitti MP, Solling TI, Schalk O, Kowalewski M, Marangos JP, Robb MA, Johnson AS, Worner HJ, Shalashilin DV, Miller RJD, Domcke W, Ueda K, Weber PM, Cireasa R, Vacher M, Roberts GM, Decleva P, Bencivenga F, Neumark DM, Gessner O, Stolow A, Mishra PK, Polyak I, Baeck KK, Kirrander A, Dowek D, Jimenez-Galan A, Martin F, Mukamel S, Sekikawa T, Gelin MF, Townsend D, Makhov DV, Neville SPet al., 2016, Electronic and non-adiabatic dynamics: general discussion, Faraday Discussions, Vol: 194, Pages: 209-257, ISSN: 1359-6640

Journal article

Robb MA, 2016, Charge migration in polycyclic norbornadiene cations: winning the race against decoherence, Journal of Chemical Physics, Vol: 145, ISSN: 1089-7690

The observation of electronic motion remains a key target in the development of the eld of attoscience.However, systems in which long-lived oscillatory charge migration may be observed must be selected carefully,particularly because it has been shown that nuclear spatial delocalization leads to a loss of coherent electrondensity oscillations. Here we demonstrate electron dynamics in norbornadiene and extended systems wherethe hole density migrates between two identical chromophores. By studying the e ect of nuclear motionand delocalization in these example systems, we present the physical properties that must be considered incandidate molecules in which to observe electron dynamics. Furthermore, we also show a key contribution tonuclear delocalization arises from motion in the branching plane of the cation. For the systems studied, thedephasing time increases with system size while the energy gap between states, and therefore the frequency ofthe density oscillation, decreases with size (obeying a simple exponential dependence on the inter-chromophoredistance). We present a system that balances these two e ects and shows several complete oscillations in thespin density before dephasing occurs.

Journal article

Vacher M, Bearpark MJ, Robb MA, 2016, Direct methods for non-adiabatic dynamics: connecting the single-set variational multi-confguration Gaussian (vMCG) and Ehrenfest perspectives, Theoretical Chemistry Accounts, Vol: 135, ISSN: 1432-881X

In this article, we outline the current state-of-theart“on-the-fly” methods for non-adiabatic dynamics, highlightingthe similarities and differences between them. Wederive the equations of motion for both the Ehrenfest andvariational multi-configuration Gaussian (vMCG) methodsfrom the Dirac–Frenkel variational principle. We explorethe connections between these two methods by presentingan alternative derivation of the vMCG method, which givesthe Ehrenfest equations of motion when taking the appropriatelimits.

Journal article

Vacher M, Albertani FEA, Jenkins AJ, Polyak I, Bearpark M, Robb MAet al., 2016, Electron and nuclear dynamics following ionisation of modified bismethylene-adamantane, Faraday Discussions, Vol: 194, Pages: 95-115, ISSN: 1364-5498

We have simulated the coupled electron and nuclear dynamics using the Ehrenfest method upon valence ionisation of modified bismethylene-adamantane (BMA) molecules where there is an electron transfer between the two π bonds. We have shown that the nuclear motion significantly affects the electron dynamics after a few fs when the electronic states involved are close in energy. We have also demonstrated how the non-stationary electronic wave packet determines the nuclear motion, more precisely the asymmetric stretching of the two π bonds, illustrating “charge-directed reactivity”. Taking into account the nuclear wave packet width results in the dephasing of electron dynamics with a half-life of 8 fs; this eventually leads to the equal delocalisation of the hole density over the two methylene groups and thus symmetric bond lengths.

Journal article

Jenkins AJ, Vacher M, Bearpark MJ, Robb MAet al., 2016, Nuclear spatial delocalization silences electron density oscillations in 2-phenyl-ethyl-amine (PEA) and 2-phenylethyl-N,N-dimethylamine (PENNA) cations (vol 144, 104110, 2016), JOURNAL OF CHEMICAL PHYSICS, Vol: 144, ISSN: 0021-9606

Journal article

Jenkins AJ, Vacher M, Bearpark MJ, Robb MAet al., 2016, Nuclear spatial delocalization silences electron density oscillations in 2-phenyl-ethyl-amine (PEA) and 2-phenylethyl-N,N-dimethylamine (PENNA) cations, Journal of Chemical Physics, Vol: 144, ISSN: 1089-7690

We simulate electron dynamics following ionization in 2-phenyl-ethyl-amine and 2-phenylethyl-N,N-dimethylamine as examples of systems where 3 coupled cationic states are involved. We study two nuclear effects on electron dynamics: (i) coupled electron-nuclear motion and (ii) nuclear spatial delocalization as a result of the zero-point energy in the neutral molecule. Within the Ehrenfest approximation, our calculations show that the coherent electron dynamics in these molecules is not lost as a result of coupled electron-nuclear motion. In contrast, as a result of nuclear spatial delocalization, dephasing of the oscillations occurs on a time scale of only a few fs, long before any significant nuclear motion can occur. The results have been rationalized using a semi-quantitative model based upon the gradients of the potential energy surfaces.

Journal article

Sanchez-Gonzalez A, Barillot TR, Squibb RJ, Kolorenc P, Agaker M, Averbukh V, Bearpark MJ, Bostedt C, Bozek JD, Bruce S, Montero SC, Coffee RN, Cooper B, Cryan JP, Dong M, Eland JHD, Fang L, Fukuzawa H, Guehr M, Ilchen M, Johnsson AS, Liekhus-S C, Marinelli A, Maxwell T, Motomura K, Mucke M, Natan A, Osipov T, Ostlin C, Pernpointner M, Petrovic VS, Robb MA, Sathe C, Simpson ER, Underwood JG, Vacher M, Walke DJ, Wolf TJA, Zhaunerchyk V, Rubensson J-E, Berrah N, Bucksbaum PH, Ueda K, Feifel R, Frasinski LJ, Marangos JPet al., 2015, Auger electron and photoabsorption spectra of glycine in the vicinity of the oxygen K-edge measured with an X-FEL, JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol: 48, ISSN: 0953-4075

Journal article

Vacher M, Steinberg L, Jenkins AJ, Bearpark MJ, Robb MAet al., 2015, Electron dynamics following photoionization: decoherence due to the nuclear-wave-packet width, Physical Review A, Vol: 92, ISSN: 1094-1622

Journal article

Santolini V, Malhado JP, Robb MA, Garavelli M, Bearpark MJet al., 2015, Photochemical reaction paths of cis-dienes studied with RASSCF: the changing balance between ionic and covalent excited states, Molecular Physics, Vol: 113, Pages: 1978-1990, ISSN: 1362-3028

The balanced description of ionic and covalent molecular excited electronic states still presents a challenge for currentelectronic structure methods. In this contribution, we show how the restricted active space self-consistent field (RASSCF)method can be used to address this problem, applied to two dienes in the cis conformation. As with the closely relatedcomplete active space self-consistent field (CASSCF) method, the construction of the orbital active space in the RASSCFmethodology requires the a priori formulation of a physical or theoretical model of the system being studied. In this article,we discuss how the active space can be constructed in a guided and systematic way, using pairs of natural bond orbitalsas correlating partner orbitals (oscillator orbitals) and semi-internal correlation. The resulting balanced description of thecovalent and ionic valence excited states – with the ionic state correctly lower in energy at the Franck–Condon geometry –is suitable to study the photochemistry of these and other molecules.

Journal article

Robb MA, Meisner J, Vacher M, Bearpark Met al., 2015, Geometric rotation of the nuclear gradient at a conical intersection: Extension to complex rotation of diabatic states, Journal of Chemical Theory and Computation, Vol: 11, Pages: 3115-3122, ISSN: 1549-9618

Nonadiabatic dynamics in the vicinity of conical intersections is of essential importance in photochemistry. It is well known that if the branching space is represented in polar coordinates, then for a geometry represented by angle θ, the corresponding adiabatic states are obtained from the diabatic states with the mixing angle θ/2. In an equivalent way, one can study the relation between the real rotation of diabatic states and the resulting nuclear gradient. In this work, we extend the concept to allow a complex rotation of diabatic states to form a nonstationary superposition of electronic states. Our main result is that this leads to an elliptical transformation of the effective potential energy surfaces; i.e., the magnitude of the initial nuclear gradient changes as well as its direction. We fully explore gradient changes that result from varying both θ and ϕ (the complex rotation angle) as a way of electronically controlling nuclear motion, through Ehrenfest dynamics simulations for benzene cation.

Journal article

Vacher M, Meisner J, Mendive-Tapia D, Bearpark MJ, Robb MAet al., 2015, Electronic Control of Initial Nuclear Dynamics Adjacent to a Conical Intersection, JOURNAL OF PHYSICAL CHEMISTRY A, Vol: 119, Pages: 5165-5172, ISSN: 1089-5639

Journal article

Li Q, Mennucci B, Robb MA, Blancafort L, Curutchet Cet al., 2015, Polarizable QM/MM Multiconfiguration Self-Consistent Field Approach with State-Specific Corrections: Environment Effects on Cytosine Absorption Spectrum, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, Vol: 11, Pages: 1674-1682, ISSN: 1549-9618

Journal article

Vacher M, Mendive-Tapia D, Bearpark MJ, Robb MAet al., 2015, Electron dynamics upon ionization: Control of the timescale through chemical substitution and effect of nuclear motion, Journal of Chemical Physics, Vol: 142, ISSN: 1089-7690

Journal article

Boggio-Pasqua M, Bearpark MJ, Rob MA, 2015, The Role of Extended Conical Intersection Seams in Photochromic Systems, International Conference of Computational Methods in Sciences and Engineering (ICCMSE), Publisher: AMER INST PHYSICS, Pages: 453-456, ISSN: 0094-243X

Conference paper

Robb MA, Robb MA, Robb MA, 2014, In This Molecule There Must Be a Conical Intersection, Advances in Physical Organic Chemistry, Vol: 48, Pages: 190-239

Journal article

Mendive-Tapia D, Perrier A, Bearpark MJ, Robb MA, Lasorne B, Jacquemin Det al., 2014, New insights into the by-product fatigue mechanism of the photo-induced ring-opening in diarylethenes, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 16, Pages: 18463-18471, ISSN: 1463-9076

Journal article

Perveaux A, Lauvergnat D, Lasorne B, Gatti F, Robb MA, Halasz GJ, Vibok Aet al., 2014, Attosecond electronic and nuclear quantum photodynamics of ozone: time-dependent Dyson orbitals and dipole, JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, Vol: 47, ISSN: 0953-4075

Journal article

Vacher M, Mendive-Tapia D, Bearpark MJ, Robb MAet al., 2014, The second-order Ehrenfest method A practical CASSCF approach to coupled electron-nuclear dynamics, THEORETICAL CHEMISTRY ACCOUNTS, Vol: 133, ISSN: 1432-881X

Journal article

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