Calculated Energy Gap Research Articles

Analysis of electronic, Raman and UV-vis spectra for Zn11Se11, Zn11S11, and ternary alloys Zn11SnSe11-n(n= 1-11) A DFT/TDDFT study.

Zn11Se11, Zn11S11, and ternary alloys from Zn11SnSe11-n (n = 1 - 11), with cubic structures represented by nanostructures called tetramantane, have been studied theoretically by investigating the electronic properties, Raman and UV-vis spectra. LUMO and HOMO levels were observed to change with the number of the sulfur atoms. The (HOMO - LUMO) gap for Zn11Se11 (2.377205eV) increased with the sulfur atoms. Zn11S8Se3 was calculated to have an energy gap (3.061305eV) less than others ternary alloys. The calculated energy gap of Zn11S11 (3.597374eV) is in a high agreement with experimental value (3.6 eV). Raman spectra for ternary alloys Zn11SnSe11-n content peaks result from the connection of (Zn11Se11+Zn11S11) peaks, Zn11Se11 has peak at 260.42 cm-1 shifts from the experimental value by a small deviasion which is produced due to the confinement effect. UV-vis spectra for ternary alloys shifted to a higher energy level with the increase in the number of sulfur atoms and dramatically close to Zn11S11 UV-vis spectrum except Zn11S8Se3 has λmax at 342 nm. These nanostructures are suitable to be used in different applications such as lenses, photoelectronic devices, solar energy, and biosensors. DFT/TDDFT at the B3LYP level with SDD basis functions is used. All the calculations are completed using the Gaussian 09 program.

Relationship between ESIPT properties and antioxidant activities of 5-hydroxyflavone derivates**Project supported by the National Basic Research Program of China (Grant No. 2019YFA0307701), the National Natural Science Foundation of China (Grant No. 11874180), and the Science and Technology Development Project of Jilin Province of China (Grant No. 20190103101JH).

It is of great significance to study the relationship between the excited state intramolecular proton transfer (ESIPT) properties and antioxidant activities of compounds in the field of life sciences. In this work, two novel compounds 5HF–OMe and 5HF–NH2 are designed through introducing a methoxy- and amino-group into the structure of 5-hydroxyflavone (5HF) respectively. The relationship between the ESIPT reaction and antioxidant activities of the three compounds is studied via the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated potential energy curves suggest that the rate of ESIPT reaction will gradually slow down from 5HF to 5HF–OMe and 5HF–NH2. In addition, the antioxidant activities of the three compounds gradually enhance from 5HF to 5HF–OMe and 5HF–NH2, which can be seen from the calculated energy gaps and ionization potential values. Interestingly, the above results imply that the rate of ESIPT reaction has a negative relationship with the antioxidant activities of the compounds, i.e., the slower rate of ESIPT reaction will reflect the higher antioxidant activity of the compound, which will provide valuable reference for detecting the antioxidant activity of compound via the photophysical method.

InP/TiO2 heterojunction for photoelectrochemical water splitting under visible-light

Hydrogen production through photoelectrochemical (PEC) water splitting on photocatalyst is a green and clean method. In this study, we use density functional theory (DFT) calculations to find that the cage-like InP quantum dots (QDs) sensitized TiO2 is an effective photocatalyst for PEC water splitting under visible-light. A 16-ps first-principle molecular dynamics (FPMD) simulation results indicate that the cage-like InP-12, InP-16, InP-20, InP-24, InP-28, and InP-36 QDs are stable at room temperature (300 K). Furthermore, the calculated energy gaps of InP-16, InP-20, InP-24, InP-28, and InP-36 QDs are about 2.0 eV, which are suitable for visible-light absorption. Stable InP-20/TiO2 heterojunction structure was also obtained by FPMD simulation, and the electronic structure calculation result indicates that the InP-20/TiO2 heterojunction has a favorable type-II band aligment, which could prevent the recombination of photoexcited carriers. Finally, the possible reaction pathways of hydrogen production on InP-20/TiO2 heterojunction were investigated. It is found that energy barrier of hydrogen production of the InP-20/TiO2 is 2.56 eV lower than pure TiO2. Our calculations imply that InP QDs sensitized anatase TiO2 is an effective photocatalyst for visible-light PEC water splitting.

Facile synthesis of accordion-like Y2O3:Er3+ nanothermometers for ratiometric temperature sensing applications

Non-contact thermometry operating at the sub-micrometer level based on self-referencing upconversion (UC) micro-nanomaterial is an attractive pursuit for optical thermometers. Here, a novel accordion-like Y2O3:Er3+ nanothermometer with green/red UC luminescence is obtained from the solvent-free grinding and the following temperature-programmed calcination of the Er3+-doped Y-containing metal-organic framework (MOF) precursor. Upon 980 nm laser irradiation, the as-prepared Y2O3:Er3+ exhibits dual-emitting green and red emissions originating from Er3+ ions with a two-photon UC process, and the intensities are strongly dependent on temperature. The optical thermometry behaviours of Y2O3:Er3+ nanothermometer using the fluorescence intensity ratio (FIR) technique are studied in the temperature range of 298–393 K within the physiological temperature, in which the thermally coupled levels of two green emissions and another pair of coupled levels with red and green emissions are comparatively investigated. A relative large calculated energy gap ΔE (951 cm−1) for the thermally couple levels of 2H11/2 and 4S3/2 and a maximum absolute sensitivity (Sa) of 0.0027 K−1 at 393 K are determined, respectively. This novel accordion-like Y2O3:Er3+ nanothermometer exhibits excellent optical sensing stability and high temperature resolution, demonstrating its great potential as reliable and promising optical thermometers.

Density Function Theory calculation, and phthalonitrile process for a synthesis of single crystal zinc phthalocyanine

This study aims to synthesize the β-phase single crystal of zinc phthalocyanine (ZnPc) by using phthalonitrile process and vapor deposition. The single-crystal X-ray diffraction was used to identify the molecular structure and lattice parameters of the synthesized material. The molecular structure, frontier molecular orbitals, and IR spectroscopy of ZnPc were investigated through Time Dependent (TD)-Density Function Theory (DFT) calculations that is compared with experimental results. For the solid-state simulation, the experimental crystallographic data was used to determine band structure and density of state using DFT method on Quantum Espresso. The β-phase monoclinic single crystal of ZnPc is as a direct band gap semiconductor with the calculated energy gap of 2.1 eV, and the Fermi energy level of 2.61 eV.

A density functional study on geometries, electronic and photo sensitized surface properties of the fullerene-like BGe nanomaterials

Geometries associated with relative stabilities, infrared spectra, and energy gaps of the slightly deformed (Ge6B6)n (n = 2–10) nanocages are systematically investigated applying density functional theory. Particularly, the relative stabilities in terms of the calculated averaged atomic binding energies and fragmentation energies show that (GeB)36 cage has enhanced stability over other nanoclusters. Interestingly, (Ge6B6)n nanocages prefer to the sphere-like geometries with large inner hollow spaces. Moreover, the calculated energy gaps of (Ge6B6)n nanocages are decreased generally and the wavelengths of absorption spectra in sphere-like (Ge6B6)n clusters are elongated with the increased size of (Ge6B6)n nanocages. The quantum confinement effect indirectly is revealed; furthermore, the nanosized (GeB)36-60 cages have a very stronger capacity for solar energy absorption or conversion due to narrow energy gaps and large DOS near LUMO and HOMO levels. Additionally, germanium atoms transfer electronic charges to their surrounding boron atoms, charge-transfers between Ge and B enhance stabilities of the nanocages. SH2 gas is sensitive to GeB cage and can be used to removal SH2 from air.

Theoretical evaluation of valeraldehyde

The aim of this study was to compute the theoretical (software-based) spectroscopic properties of valeraldehyde as a member of the aldehyde family. The structural, thermochemical, electrical and spectroscopic properties of valeraldehyde were investigated using a quantum calculation approach. The infrared, ultraviolet-visible, Raman, and vibrational self-consistent field calculations and thermochemistry were calculated using the computational software GAMESS (General Atomic and Molecular Electronic Structure System) and the nuclear magnetic resonance predictions were calculated. The calculated energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) was 164.892.eV, which means valeraldehyde is a poor electrical conductor. The band gap is a non-neglected parameter for optical material. Results of the current computational analysis are useful to predict even a complex aldehyde precursor.
 Significance:
 
 The computational strategy used successfully here could also be useful for investigation of other aldehyde polymers.

Stability and electronic properties of IrnV (n = 2–10) nanoclusters and their reactivity toward N2H4 molecule

DFT calculations have been carried out over the IrnV (n = 2–10) clusters in order to predict their stability, electronic, and catalytic properties. Based on the fragmentation energy (ΔEf) and the second-order difference energy (Δ2E), the results show that the V-doped iridium clusters with sizes of n = 5 and 7 were found more stable than their neighboring clusters. Our results also exhibit that the calculated energy gaps (Eg) for these clusters are in the range 0.074–0.603 eV, suggesting that the metallic character can manifest in these binary clusters. Therefore, the IrnV clusters can be utilized as catalysts in several catalytic reactions. MEP analysis exhibits that the highest positive charge resides on the V atom, thus the V atom in the binary clusters can be considered as the most active site for the nucleophilic attack. So, it represents a favorable electrophilic adsorption site that can strongly interact with the electron-rich molecules. The influence of the N2H4 adsorption over the electronic properties of IrnV clusters was also investigated. The results show that the adsorption energies (Eads) vary between − 21.3 and − 46.6 kcal mol−1, suggesting a chemisorption process. The change in enthalpy (ΔHads) is in the range of − 24.6 to − 51.0 kcal mol−1, indicating that the interaction between the IrnV clusters and the N2H4 molecule is very strong, and the formed complexes by chemisorption between the clusters and the N2H4 molecule are thermodynamically stable at standard conditions. The energy gaps of clusters are sharply changed upon adsorption of hydrazine onto the surface of the clusters, implying a great sensitivity of these clusters toward N2H4 molecule. The calculated dipole moments were also largely increased after chemisorption of the N2H4 molecule over the clusters.

Neutron-proton pairing in nuclear matter

The self-energy effect on the neutron-proton (np) pairing gap is investigated up to the third order within the framework of the extend Bruecker-Hartree-Fock (BHF) approach combined with the BCS theory. The self-energy up to the second-order contribution turns out to reduce strongly the effective energy gap, while the \emph{renormalization} term enhances it significantly. In addition, the effect of the three-body force on the np pairing gap is shown to be negligible. To connect the present results with the np pairing in finite nuclei, an effective density-dependent zero-range pairing force is established with the parameters calibrated to the microscopically calculated energy gap.

Effect of the formation of silver nano‐binuclear complex on the refinement of cytotoxic and antibacterial potency of coumarin Schiff base: Spectroscopy, thermal, X‐ray diffraction analyses and density functional theory calculations

A new Schiff base (SCH) was prepared using 8‐acetyl‐7‐hydroxy‐4‐methylcoumarin and sulfaclozine. Its silver complex was also synthesized. These compounds were subjected to elemental and spectral analyses. The silver complex was also subjected to thermogravimetric analysis, molar conductance measurements, scanning electron microscopy, energy‐dispersive X‐ray analysis and X‐ray diffraction measurements. The formula of the complex was determined as [(Ag2SCH(NO3)2]. The molar conductance indicates its non‐electrolyte character. The electronic structure and nonlinear optical parameters of SCH and its complex were investigated theoretically at the B3LYP/GENECP level. The geometry of the studied complex is non‐planar as indicated from the dihedral angles between central Ag metal ion and coordinated centres of SCH. Density functional theory calculations were also carried out to calculate the global properties of hardness, global softness and electronegativity. The calculated energy gap between HOMO and LUMO energies shows that charge transfer occurs within the studied complex. The total static dipole moment, the mean polarizability, the anisotropy of the polarizability and the mean first‐order hyperpolarizability were calculated and compared with those of urea as a reference material. The results of the mean first‐order hyperpolarizability showed that SCH and its silver complex are excellent candidates as nonlinear optical materials. The compounds were screened for their antimicrobial effect against various species of bacteria and fungi. Their anticancer activities were evaluated in vitro against the human cancer cell line MCF‐7 (human breast adenocarcinoma).

Physico-Chemical Insights into Gas-Phase and Oxide-Supported Sub-Nanometre AuCu Clusters

Abstract Catalysis by AuCu nanoclusters is a promising scientific field. However, our fundamental understanding of the underlying mechanisms of mixing in AuCu clusters at the sub-nanometre scale and their physico-chemical properties in both the gas-phase and on oxide supports is limited. We have identified the global minima of gas-phase and MgO(100)-supported AuCu clusters with 3–10 atoms using the Mexican Enhanced Genetic Algorithm coupled with density functional theory. Au and Cu adatoms and supported dimers have been also simulated at the same level of theory. The most stable composition, as calculated from mixing and binding energies, is obtained when the Cu proportion is close to 50%. The structures of the most stable free AuCu clusters exhibit Cu-core/Au-shell segregation. On the MgO surface however, there is a preference for Cu atoms to lie at the cluster-substrate interface. Due to the interplay between the number of interfacial Cu atoms and surface-induced cluster rearrangement, on the MgO surface 3D structures become more stable than 2D structures. The O-site of MgO surface is found to be the most favourable adsorption site for both metals. All dimers favour vertical (V) configurations on the surface and their adsorption energies are in the order: AuCu < CuCu < AuAu < AuCu (where the underlined atom is bound to the O-site). For both adatoms and AuCu dimers, adsorption via Cu is more favourable than Au-adsorbed configurations, but, this disagrees with the ordering for the pure dimers due to a combination of electron transfer and the metal-on-top effect. Binding energy (and second difference) and HOMO-LUMO gap calculations show that even-atom (even-electron) clusters are more stable than the neighbouring odd-atom (odd- electron) clusters, which is expected for closed- and open-shell systems. Supporting AuCu clusters on the MgO(100) surface decreases the charge transfer between Au and Cu atoms calculated in free clusters. The results of this study may serve as a foundation for designing better AuCu catalysts.

Quantum chemical approach for chemiluminescence characteristics of di-substituted luminol derivatives in polar solvents

The presence of electron donating substituents altered the electronic properties of the luminol and this phenomenon was evaluated in relation to the chemiluminescence potential of the studied disubstituted luminol derivatives using three different theoretical calculation methods: Restricted Hartree Fork (RHF), Density Functional Theory (DFT) with hybrid function of Becke three-parameters Lee, Yang and Parr (B3LYP), i.e. (DFT-B3LYP), and Møller-Plesset Perturbation Theory (MP2). Different behavior was observed for the studied molecular structures in different phases (gas, water, and dimethylformamide) to emphasize the influence of molecular environment on the chemiluminescence property, regardless of the theoretical methods employed. Time-dependent DFT with correction terms enabled accurate prediction of frontier orbital energies such that calculated energy gaps could reasonably correlate with the lowest excitation energies. Besides, the electron donating moiety induced better chemiluminescence property depending on the donating power, position as well as the number of such substituents. Dimethylamino-substituted luminol showed better quantum chemical parameters and spectra pattern to have much-enhanced chemiluminescence properties possibly. Specifically, its positional isomer named 6,8-bis(dimethylamino)-5-amino-2,3-dihydro-1,4-phthalazinedione showed a greater potential of being a better chemiluminescence reagent in various analytical assays.

A theoretical study of the geometries, and electronic and surface properties of sphere-like (SiB)2n (n = 6-27, 30) functional nanomaterials.

The geometries and electronic properties of (SiB)2n (n = 6-27, 30) clusters are systematically investigated based on the gradient corrected Perdew-Burke-Ernzerhof exchange-correlation functional. In particular, the (SiB)36 cage is identified as the most stable nanocluster and (SiB)2n (n = 6-27, 30) nanocages prefer to have sphere-like geometries. By increasing the (SiB)2n (n = 6-27, 30) nanocage size, the calculated energy gaps of (SiB)2n (n = 6-27, 30) nanocages generally decrease and absorption wavelengths of the spectra of (SiB)2n (n = 6-27, 30) nanoclusters are elongated. The varied size of the nanoclusters leads to a quantum confinement effect indirectly. Interestingly, the nanosized (SiB)30-60 cages exhibit a stronger capacity for solar energy absorption or conversion due to both narrow HOMO-LUMO energy gaps and a large DOS near LUMO and HOMO levels. Finally, electronic charges transferred from silicon atoms to their surrounding boron atoms in (SiB)2n (n = 6-27, 30) contribute to the metallic characteristic and B-Si ionic bonds, and eventually enhance the stabilities of the nanocages.

Synthesis, characterization, and theoretical studies of (E)-t-butyl-2-((E)-2-methyl-3-phenylallylidene) hydrazine carboxylate and (E)-t-butyl-2-((E)-3-phenylallylidene) hydrazine carboxylates as a possible Mcl-1 antagonists

Refluxing t-butyl carbazate with the corresponding aldehyde in ethanol yielded (E)-tert-butyl 2-((E)-2-methyl-3-phenylallylidene)hydrazinecarboxylate (1) and (E)-t-butyl-2-((E)-3-phenylall ylidene)hydrazine carboxylate (2) in appreciable amounts. The products were identified using spectral data analysis. To confirm the trans-nature of the double bond in the synthesized compound 1, single crystal X-ray analysis was carried out. The trans-nature of double bond in compound 2, which could not be crystallized, was however deduced from the vicinal coupling constant of the two protons involved in the double bond. Single crystal X-ray study unfurled that compound 1 crystallizes in a monoclinic system with P21/c space group. Theoretical calculations of both the compounds was carried out at DFT/B3LYP/6-311G (d, p) level of theory. Theoretically obtained parameters demonstrated good compliance with the experimentally obtained data for compound 1. Molecular Electrostatic Potential Surface (MEPS) and Frontier Molecular Orbital (FMO) analysis were performed for both the compounds. Calculation of HOMO-LUMO energy gap and various other related parameters depicted a similar reactivity behaviour of both the compounds. To unravel the biological target of the compounds, Online Pass Prediction divulged that compounds, 1 and 2 are moderately potent against Mcl-1 enzyme, with Pa value of 0.82 and 0.61 respectively. Molecular docking of 1 and 2 against Mcl-1 protein conferred binding free energy of −6.26 and −3.11 kcal/mol respectively indicating moderate binding efficiency of the compounds.

Temperature dependence of the energy band gap of CuSi2P3 semiconductor using PSOPW method

Abstract Theoretical formalism based on the orthogonalized plane wave method supplemented by a potential scaling scheme was used to predict the temperature dependence of energy gap of CuSi2P3 semiconductor. A computer code in Pascal was used to perform the variation of fundamental energy gap with temperature in the range of 150 K to 800 K. The dependence of energy gap on temperature for lattice dilation contribution, lattice vibration contribution and total temperature effect were performed separately. The results revealed that, as temperature increases, the top of the valence band and the bottom of the conduction band increase, while the energy band gap decreases. Generally, at low temperatures, the energy gap varies slowly and exhibits a nonlinear dependence and approaches linearity as temperature increases. The calculated energy gap of CuSi2P3 at T = 300 K is 0.4155 eV. The temperature coefficients in the linear region due to lattice dilation contribution, lattice vibration contribution and total temperature effect were calculated as –1.101 × 10−5 eV/K, –1.637 × 10−4 eV/K and –1.7523 × 10−4 eV/K, respectively. Also, the ratio of temperature coefficient of the energy gap due to LV contribution to its value and LD contribution in the linear region is equal to 14.868. That ratio is compared to those of CuGe2P3 and III-V compounds, where those of the latter show a systematic change with Eg. Moreover, the Eg of all the compounds shows a quadratic dependence on the inverse of mean bond length.

Mixed ab initio and semiempirical study of hydrogen-terminated finite germanium nanowires

We present a mixed ab initio and semiempirical method for the cohesive energy and electronic gap calculations of hydrogen passivated tetrahedral and clathrate germanium nanowires (∼ 1850 atoms) with acceptable accuracy, comparable to density functional theory results, and with a significantly lower computational cost. First, we find that the PM6 semiempirical method produce the most accurate geometries when compared with the DFT results; whereas other semiempirical methods such as AM1, PM3 and PM7 clearly underestimate (or overestimate). Second, we implement the DFT@PM6 mixed scheme for cohesive/binding energy and electronic gap calculations which shows promising results compared with reference values of DFT. However, the bulk energy gap and binding energy values from the quantum confinement fitting procedure slightly underestimate the results which can be easily overcome using suitable functional and basis set/ECP. Also, a comparison with previous work clearly shows that the calculated electronic gap for bulk germanium is extremely sensitive to the choice of framework. Further development in this research work is progressing.

Luminescence light collection technology in the aragonite of stone corals

Stone corals do not use calcium carbonate in the form of calcite, which has a calculated energy gap of 3.93 eV, but in the form of aragonite, which has a calculated energy gap of 2.88 eV (here experimentally determined to amount to 2.46 eV) as a building material. This enables the coral to harvest blue light, which is penetrating and filtering deep into the surface water of the ocean. White luminescence, which is composed of different wave lengths, is generated, and then conducted and redistributed within the aragonite structure to be supplied to the symbiotic photosynthetic algae. This mechanism of light concentration and adaptation via the aragonite structure leads to a smaller amount of light-absorbing phosphorescent pigments being required for the symbionts to survive. The resulting advantages are a significantly smaller exposure to photo-degradation, less effort for chemical synthesis and higher efficiency for solar energy conversion. The mechanism of luminescence light collection in the aragonite network is discussed on the basis of spectroscopic measurements on thin coral slices and in context with its architecture and the coral’s living activities. The relevance of the stone coral’s fractal geometry, both for broadband solar light harvesting and luminescence light collection within non-imaging optics, is emphasized. It is proposed that synthetic aragonite should be developed as a solar energy material.

Energy-Resolved Photoconductivity Mapping in a Monolayer-Bilayer WSe2 Lateral Heterostructure.

Vertical and lateral heterostructures of van der Waals materials provide tremendous flexibility for band-structure engineering. Because electronic bands are sensitively affected by defects, strain, and interlayer coupling, the edge and heterojunction of these two-dimensional (2D) systems may exhibit novel physical properties, which can be fully revealed only by spatially resolved probes. Here, we report the spatial mapping of photoconductivity in a monolayer-bilayer WSe2 lateral heterostructure under multiple excitation lasers. As the photon energy increases, the light-induced conductivity detected by microwave impedance microscopy first appears along the heterointerface and bilayer edge, then along the monolayer edge, inside the bilayer area, and finally in the interior of the monolayer region. The sequential emergence of mobile carriers in different sections of the sample is consistent with the theoretical calculation of local energy gaps. Quantitative analysis of the microscopy and transport data also reveals the linear dependence of photoconductivity on the laser intensity and the influence of interlayer coupling on carrier recombination. Combining theoretical modeling, atomic-scale imaging, mesoscale impedance microscopy, and device-level characterization, our work suggests an exciting perspective for controlling the intrinsic band gap variation in 2D heterostructures down to a regime of a few nanometers.

Porous polydimethylsiloxane membranes loaded with low-temperature crystallized TiO2 NPs for detachable antibacterial films

TiO2 was prepared via sol–gel synthesis, and in order to obtain crystallized anatase in a colloidal suspension, it was subjected to a hydrothermal process in the presence of a minimum amount of nanoparticles, which acted as nucleation seeds. Concurrently, high-pH sol–gel syntheses were performed with the increase in NH3 solutions as a comparison for the samples’ activity. Polydimethylsiloxane was chosen as supporting material for the TiO2 nanoparticles for its high surface area and its stability against catalyst’s photoactivity. They were both processed with the electrospinning technique: It was possible to disperse titania among PDMS fibers and to obtain detachable polymeric membranes with the appearance of white soft sheets. These membranes show antibacterial activity under fluorescent light, and thus, they may be used as a film for several kinds of surfaces. The synthesized samples were controlled and characterized by means of rheological measurements, scanning electron microscopy images together with EDS maps, differential scanning calorimetry analyses, diffuse reflectance spectra for energy gap (Egap) calculations and recycling photocatalytic experiments, while Escherichia Coli inactivation gave evidence of the antibacterial activity.

Quantum Monte Carlo calculations of energy gaps from first principles

We review the use of continuum quantum Monte Carlo (QMC) methods for the calculation of energy gaps from first principles, and present a broad set of excited-state calculations carried out with the variational and fixed-node diffusion QMC methods on atoms, molecules, and solids. We propose a finite-size-error correction scheme for bulk energy gaps calculated in finite cells subject to periodic boundary conditions. We show that finite-size effects are qualitatively different in two-dimensional materials, demonstrating the effect in a QMC calculation of the band gap and exciton binding energy of monolayer phosphorene. We investigate the fixed-node errors in diffusion Monte Carlo gaps evaluated with Slater-Jastrow trial wave functions by examining the effects of backflow transformations, and also by considering the formation of restricted multideterminant expansions for excited-state wave functions. For several molecules, we examine the importance of structural relaxation in the excited state in determining excited-state energies. We study the feasibility of using variational Monte Carlo with backflow correlations to obtain accurate excited-state energies at reduced computational cost, finding that this approach can be valid. We find that diffusion Monte Carlo gap calculations can be performed with much larger time steps than are typically required to converge the total energy, at significantly diminished computational expense, but that in order to alleviate fixed-node errors in calculations on solids the inclusion of backflow correlations is sometimes necessary.