A Quantum Computing Implementation of Nuclear-Electronic Orbital (NEO) Theory: Towards an Exact pre-Born-Oppenheimer Formulation of Molecular Quantum Systems

Nuclear quantum phenomena beyond the Born-Oppenheimer approximation are known to play an important role in a growing number of chemical and biological processes. While there exists no unique consensus on a rigorous and efficient implementation of coupled electron-nuclear quantum dynamics, it is recognised that these problems scale exponentially with system size on classical processors and therefore may benefit from quantum computing implementations. Here, we introduce a methodology for the efficient quantum treatment of the electron-nuclear problem on near-term quantum computers, based upon the Nuclear-Electronic Orbital (NEO) approach. We generalize the electronic two-qubit tapering scheme to include nuclei by exploiting symmetries inherent in the NEO framework; thereby reducing the hamiltonian dimension, number of qubits, gates, and measurements needed for calculations. We also develop parameter transfer and initialisation techniques, which improve convergence behavior relative to conventional initialisation. These techniques are applied to H$_2$ and malonaldehyde for which results agree with Nuclear-Electronic Orbital Full Configuration Interaction and Nuclear-Electronic Orbital Complete Active Space Configuration Interaction benchmarks for ground state energy to within $10^{-6}$ Ha and entanglement entropy to within $10^{-4}$. These implementations therefore significantly reduce resource requirements for full quantum simulations of molecules on near-term quantum devices while maintaining high accuracy.Comment: 26 pages, 7 figures, 10 table

Why Do Some Molecules Form Hydrates or Solvates?

The discovery of solvates (crystal structures where the solvent is incorporated into the lattice) dates back to the dawn of chemistry. The phenomenon is ubiquitous, with important applications ranging from the development of pharmaceuticals to the potential capture of CO2 from the atmosphere. Despite this interest, we still do not fully understand why some molecules form solvates. We have employed molecular simulations using simple models of solute and solvent molecules whose interaction parameters could be modulated at will to access a universe of molecules that do and do not form solvates. We investigated the phase behavior of these model solute–solvent systems as a function of solute–solvent affinity, molecule size ratio, and solute concentration. The simulations demonstrate that the primary criterion for solvate formation is that the solute–solvent affinity must be sufficient to overwhelm the solute–solute and solvent–solvent affinities. Strong solute–solvent affinity in itself is not a sufficient condition for solvate formation: in the absence of such strong affinity, a solvate may still form provided that the self-affinities of the solute and the solvent are weaker in relative terms. We show that even solvent-phobic molecules can be induced to form solvates by virtue of a pΔV potential arising either from a more efficient packing or because of high pressure overcoming the energy penalty

Ir Transition Moments of 1,3-Dimethyluracil - Linear Dichroism Measurements and Ab-Initio Calculations

The vibrational transition moment directions of 1,3-dimethyluracil (DMU), 1,3-dimethyluracil-5-d (DMU-5-d), and 1,3-dimethyluracil-6-d (DMU-6-d) have been determined from measurements of polarized IR spectra on samples partially aligned in stretched poly(vinyl alcohol) and by ab initio calculations at the HF/6-31G* level of approximation. The agreement between experiments and theory is good. IR spectra of DMU, DMU-5-d, and DMU-6-d in KBr and in D2O form, together with the calculated spectra and the IR polarizations, the basis for some reassignments. Special attention is paid to the double-bond region, where the high-frequency carbonyl stretching mode is assigned to an in-phase vibration of the C4O and the C2O. This vibration is polarized in a direction almost bisecting the angle between the two carbonyl bonds, in agreement with an in-phase assignment. The low-frequency mode is the out-of-phase vibration of the two carbonyl groups, and it is consequently polarized perpendicular to the high-frequency mode. In order to determine the orientation axis of DMU in the polymer matrix, the UV linear dichroism (LD) spectrum was measured and the polarizations of the electronic transitions were estimated from a semiempirical self-consistent reaction field (SCRF) calculation including solvent effects. The combined information from IR LD and UV LD shows that the molecular orientation axis in the stretched film experiment is parallel to the first pi --> pi* transition in DMU

Ir Transition Moments of 1,3-Dimethyluracil - Linear Dichroism Measurements and Ab-Initio Calculations

The vibrational transition moment directions of 1,3-dimethyluracil (DMU), 1,3-dimethyluracil-5-d (DMU-5-d), and 1,3-dimethyluracil-6-d (DMU-6-d) have been determined from measurements of polarized IR spectra on samples partially aligned in stretched poly(vinyl alcohol) and by ab initio calculations at the HF/6-31G* level of approximation. The agreement between experiments and theory is good. IR spectra of DMU, DMU-5-d, and DMU-6-d in KBr and in D2O form, together with the calculated spectra and the IR polarizations, the basis for some reassignments. Special attention is paid to the double-bond region, where the high-frequency carbonyl stretching mode is assigned to an in-phase vibration of the C4O and the C2O. This vibration is polarized in a direction almost bisecting the angle between the two carbonyl bonds, in agreement with an in-phase assignment. The low-frequency mode is the out-of-phase vibration of the two carbonyl groups, and it is consequently polarized perpendicular to the high-frequency mode. In order to determine the orientation axis of DMU in the polymer matrix, the UV linear dichroism (LD) spectrum was measured and the polarizations of the electronic transitions were estimated from a semiempirical self-consistent reaction field (SCRF) calculation including solvent effects. The combined information from IR LD and UV LD shows that the molecular orientation axis in the stretched film experiment is parallel to the first pi --> pi* transition in DMU

Assignment of electronic transition moment directions of adenine from linear dichroism measurements

The electronic spectra of a series of adenine derivatives have been investigated with respect to the number of electronic transitions, their intensities, and transition moments. The experimental work includes linear dichroism (LD) measurements on samples partially oriented in stretched polymer poly(vinyl alcohol) films, fluorescence anisotropy (FA), and: magnetic circular dichroism (MCD). The UV spectra of both 7-methyladenine (7MA) and 9-methyladenine (9MA) are resolved into contributions from five pi --> pi* transitions (I-V). Their polarizations relative to the C-4-C-5 axis are for 7MA +45 degrees (I, 36 600 cm(-1)), -16 degrees (II, 39 500 cm(-1)), -28 degrees (III, 42 600 cm(-1)), +76 degrees (IV 45 800 cm(-1)) and similar to-29 degrees (V, similar to 47 700 cm(-1)); and for 9MA +66 degrees (I, 36 700 cm(-1)), +19 degrees (II, 38 820 cm(-1)), -15 degrees (III, 43 400 cm(-1)), -21 degrees (IV, 46 800 cm(-1)), and similar to-64 degrees (V, similar to 48 320 cm(-1)). The experimental results are correlated with results from semiempirical INDO/S and ab initio CIS/6-31G(d) and CASPT2 molecular orbital calculations. The transition moments are found to be conserved when the adenine amino group has been substituted with either one or two methyl groups. In addition, LD and MCD spectra for 6-(dimethylamino)-9-ethylpurine, which is soluble in nonpolar solvents, have been measured in stretched polyethylene film and cyclohexane, respectively. The results indicate that the electronic transition moments of the 9-substituted adenine chromophore are essentially the same in a polar and a nonpolar solvent. On the basis of the. results,for 7MA and 9MA, the reduced LD and absorption spectra of adenine are analyzed in terms of contributions from the 7-H and 9-H tautomers of adenine. By comparison with theoretical and experimental results for purine and 2-aminopurine regarding the position of the lowest n --> pi* transition, we are able to confidently position the lowest n --> pi* transition in 9MA very close to the lowest pi --> pi* transition. The proximity of the first (1)n pi* and (1) pi pi* states in adenine might be related to the efficient nonradiative deactivation of the lowest excited (1) pi pi* state