About

Professor Walter G. Chapman is the William W. Akers Professor of Chemical and Biomolecular Engineering at Rice University. He earned his B.S. degree in Chemical Engineering from Clemson University and his Ph.D. in Chemical Engineering from Cornell University. Professor Chapman leads a research group focused on chemical and biomolecular engineering, mentoring graduate students and collaborating with various researchers and institutions. His group includes graduate students with diverse backgrounds in chemical engineering, mechanical engineering, and materials science from institutions around the world. Professor Chapman's extensive mentorship history includes numerous former graduate students and postdoctoral researchers who have gone on to careers in academia, industry, and national laboratories. He collaborates with senior scientists and professors both within Rice University and at external research organizations such as Oak Ridge National Laboratory. His professional network and research group reflect a strong commitment to advancing chemical and biomolecular engineering through education, research, and collaboration.

Research topics

• Organic chemistry
• Chemical physics
• Chemistry
• Thermodynamics
• Physics
• Materials science
• Computational chemistry
• Petroleum engineering
• Nuclear magnetic resonance
• Chemical engineering
• Geology

Selected publications

• The Protein Force Field Plays a Crucial Role in Obtaining Accurate Macromolecular Ensembles of IDPs

  Journal of Chemical Theory and Computation · 2026-01-02

  articleSenior authorCorresponding

  Atomistic models of water with increased dispersion interactions (e.g., OPC, TIP4P-D) have been proposed to produce extended conformational ensembles of intrinsically disordered proteins (IDPs). However, the role of the protein force field in obtaining accurate macromolecular ensembles of IDPs remains unclear. Isolating the influence of the protein and water models by comparison to an experimental measure (such as X-ray scattering) requires an atomic consideration of the hydration layer waters around a thermally fluctuating protein. To enable an atomically detailed scattering calculation around a thermally fluctuating solute, we have developed a new scattering model termed small and wide angle X-ray scattering for all molecular dynamics engines (SWAXS-AMDE). SWAXS-AMDE can handle the trajectory files from all of the popular molecular dynamics (MD) simulation softwares, thus facilitating a straightforward validation of force field improvements. SWAXS-AMDE computed scattering profiles for polyampholyte peptides show the AMBER ff19SB protein force field to play a crucial role in obtaining accurate macromolecular ensembles of both folded proteins and IDPs.

  PublisherDOI

• Extended molecular eigenmodes treatment of dipole–dipole NMR relaxation in real fluids

  The Journal of Chemical Physics · 2025-11-12 · 3 citations

  article

  Traditional models of NMR relaxation fail to account for the complex, multi-exponential behavior of the autocorrelation function in realistic systems characterized by soft-interactions and molecules that are chemically and physically complex. Here, we describe the relative diffusion of the spin dipoles by means of a Fokker-Planck equation that includes an interaction potential of mean force to account for the response of the physical/chemical environment around the dipoles. By numerically solving the Fokker-Planck equation for the diffusion propagator, we estimate dipole-dipole NMR relaxation for like- and unlike-spin systems via its eigenmode solution. We test the model against molecular simulations of diffusing dipoles with harmonic potentials and also validate using experimental longitudinal relaxation data from real systems, including Gd(III)-aqua and Gd(III)-DO3A-butrol complexes, the latter being an important MRI contrast agent. Using this novel approach, we predict both the inner- and outer-shell contributions to the relaxivity rates with excellent accuracy at frequencies relevant to MRI. We also show that, under the appropriate assumptions, our framework naturally recovers the Bloembergen-Purcell-Pound, the Solomon-Bloembergen-Morgan, and the Hwang-Freed models. Our implementation is general and publicly available for application to a broad range of systems.

  PublisherDOI

• Molecular-Level Insights into the NMR Relaxivity of Gadobutrol Using Quantum and Classical Molecular Simulations

  Chemical & Biomedical Imaging · 2025-04-09 · 5 citations

  articleOpen accessCorresponding

  MRI is an indispensable diagnostic tool in modern medicine; however, understanding the molecular-level processes governing NMR relaxation of water in the presence of MRI contrast agents remains a challenge, hindering the molecular-guided development of more effective contrast agents. By using quantum-based polarizable force fields, the first-of-its-kind molecular dynamics (MD) simulations of Gadobutrol are reported where the 1H NMR longitudinal relaxivity r1 of the aqueous phase is determined without any adjustable parameters. The MD simulations of r1 dispersion (i.e., frequency dependence) show good agreement with measurements at frequencies of interest in clinical MRI. Importantly, the simulations reveal key insights into the molecular level processes leading to r1 dispersion by decomposing the NMR dipole–dipole autocorrelation function G(t) into a discrete set of molecular modes, analogous to the eigenmodes of a quantum harmonic oscillator. The molecular modes reveal important aspects of the underlying mechanisms governing r1, such as its multiexponential nature and the importance of the second eigenmodal decay. By simply analyzing the MD trajectories on a parameter-free approach, the Gadobutrol simulations show that the outer-shell water contributes ∼50% of the total relaxivity r1 compared to the inner-shell water, in contrast to simulations of (nonchelated) gadolinium-aqua where the outer shell contributes only ∼15% of r1. The deviation between simulations and measurements of r1 below clinical MRI frequencies is used to determine the low-frequency electron-spin relaxation time for Gadobutrol, in good agreement with independent studies.

  PublisherDOI

• Characterization of kerogen nanopores using 2D NMR relaxation and MD simulations

  Magnetic Resonance Letters · 2025-05-30 · 2 citations

  articleOpen access

  The characterization of kerogen nanopores is crucial for predicting the geostorage capacity and recoverability of natural gas in unconventional gas shale reservoirs. Towards this end, a powerful technique is presented which integrates 2D NMR T 1 - T 2 relaxation measurements with molecular dynamics (MD) simulations of hydrocarbons confined in the nanopores of kerogen. The integrated NMR-MD technique is demonstrated using T 1 - T 2 measurements of kerogen isolates and organic-rich chalks saturated with heptane, together with MD simulations of heptane completely dissolved in a realistic kerogen model. The NMR-MD results are used to extract the swelling ratio and nanopore size distribution of kerogen as a function of depth in the reservoir. The effects of organic nanoconfinement on the T 1 relaxation dispersion and T 2 residual dipolar coupling of heptane are investigated, as well as the effect of downhole effective stress on the kerogen nanopore size as a function of depth and compaction. Potential applications in partially depleted gas shale reservoirs are discussed, including CO 2 utilization/geostorage, geostorage of green H 2 , and integration of the NMR-MD technique with thermodynamic models for predicting the competitive sorption of gas mixtures in kerogen. • MD simulations calibrate the T 2 surface relaxivity of heptane completely dissolved in kerogen, without any adjustable parameters. • Nanopore size distribution of kerogen determined from T 1 - T 2 measurements of nanopore heptane, without requiring additional experiments. • Swelling ratio of kerogen from nanopore heptane determined as a function of thermal maturity and reservoir depth. • Kerogen nanopore size decreases with reservoir depth due to increase in downhole effective stress and compaction.

  PublisherDOI

• PC-SAFT modeling of the dual, liquid–liquid equilibrium phase behavior for the Kraft Lignin–Ethanol–Water system

  Fluid Phase Equilibria · 2025-08-05

  article
  PublisherDOI

• Quantifying the Errors Introduced by Continuum Scattering Models on the Inferred Structural Properties of Proteins

  arXiv (Cornell University) · 2024-04-10

  preprintOpen accessSenior author

  Atomistic force fields that are tuned to describe folded proteins predict overly compact structures for intrinsically disordered proteins (IDPs). To correct this, improvements in force fields to better model IDPs are usually paired with scattering models for validation against experiments. For scattering calculations, protein configurations from all-atom simulations are used within the continuum-solvent model CRYSOL for comparison with experiments. To check this approach, we develop an equation to evaluate the radius of gyration (Rg) for any defined inner-hydration shell thickness given all-atom simulation data. Rg based on an explicit description of hydration waters compares well with the reference value of Rg obtained using Guinier analysis of the all-atom scattering model. However, these internally consistent estimates disagree with Rg from CRYSOL for the same definition of the inner-shell. CRYSOL can over-predict Rg by up to 2.5 Angstroms. We rationalize the reason for this behavior and highlight the consequences for force field design.

  PublisherOA PDFDOI

• Molecular Modes Elucidate the Nuclear Magnetic Resonance Relaxation of Viscous Fluids

  The Journal of Physical Chemistry B · 2024-08-09 · 8 citations

  articleOpen access

  The Bloembergen, Purcell, and Pound (BPP) theory of nuclear magnetic resonance (NMR) relaxation in fluids dating back to 1948 continues to be the linchpin in interpreting NMR relaxation data in applications ranging from characterizing fluids in porous media to medical imaging (MRI). The BPP theory is founded on assuming molecules are hard spheres with 1H–1H dipole pairs reorienting randomly; assumptions that are severe in light of modern understanding of liquids. Nevertheless, it is intriguing to this day that the BPP theory was consistent with the original experimental data for glycerol, a hydrogen-bonding molecular fluid for which the hard-sphere-rigid-dipole assumption is inapplicable. To better understand this incongruity, atomistic molecular simulations are used to compute 1H NMR T1 relaxation dispersion (i.e., frequency dependence) in two contrasting cases: glycerol, and a (non hydrogen-bonding) viscosity standard. At high viscosities, simulations predict distinct functional forms of T1 for glycerol compared to the viscosity standard, in agreement with modern measurements, yet both in contrast to BPP theory. The cause of these departures from BPP theory is elucidated, without assuming any relaxation models and without any free parameters, by decomposing the simulated T1 response into dynamic molecular modes for both intramolecular and intermolecular interactions. The decomposition into dynamic molecular modes provides an alternative framework to understand the physics of NMR relaxation for viscous fluids.

  PublisherOA PDFDOI

• Theory and modeling of molecular modes in the NMR relaxation of fluids

  The Journal of Chemical Physics · 2024-02-09 · 9 citations

  articleOpen access

  Traditional theories of the nuclear magnetic resonance (NMR) autocorrelation function for intra-molecular dipole pairs assume a single-exponential decay, yet the calculated autocorrelation of realistic systems displays a rich, multi-exponential behavior, resulting in anomalous NMR relaxation dispersion (i.e., frequency dependence). We develop an approach to model and interpret the multi-exponential intra-molecular autocorrelation using simple, physical models within a rigorous statistical mechanical development that encompasses both rotational diffusion and translational diffusion in the same framework. We recast the problem of evaluating the autocorrelation in terms of averaging over a diffusion propagator whose evolution is described by a Fokker-Planck equation. The time-independent part admits an eigenfunction expansion, allowing us to write the propagator as a sum over modes. Each mode has a spatial part that depends on the specified eigenfunction and a temporal part that depends on the corresponding eigenvalue (i.e., correlation time) with a simple, exponential decay. The spatial part is a probability distribution of the dipole pair, analogous to the stationary states of a quantum harmonic oscillator. Drawing inspiration from the idea of inherent structures in liquids, we interpret each of the spatial contributions as a specific molecular mode. These modes can be used to model and predict the NMR dipole-dipole relaxation dispersion of fluids by incorporating phenomena on the molecular level. We validate our statistical mechanical description of the distribution in molecular modes with molecular dynamics simulations interpreted without any relaxation models or adjustable parameters: the most important poles in the Padé-Laplace transform of the simulated autocorrelation agree with the eigenvalues predicted by the theory.

  PublisherOA PDFDOI

• Effect of Nanoconfinement on NMR Relaxation of Heptane in Kerogen from Molecular Simulations and Measurements

  The Journal of Physical Chemistry Letters · 2023-01-24 · 15 citations

  articleOpen access

  Kerogen-rich shale reservoirs will play a key role during the energy transition, yet the effects of nanoconfinement on the NMR relaxation of hydrocarbons in kerogen are poorly understood. We use atomistic MD simulations to investigate the effects of nanoconfinement on the 1H NMR relaxation times T1 and T2 of heptane in kerogen. In the case of T1, we discover the important role of confinement in reducing T1 by ∼3 orders of magnitude from that of bulk heptane, in agreement with measurements of heptane dissolved in kerogen from the Kimmeridge Shale, without any models or free parameters. In the case of T2, we discover that confinement breaks spatial isotropy and gives rise to residual dipolar coupling which reduces T2 by ∼5 orders of magnitude from the value for bulk heptane. We use the simulated T2 to calibrate the surface relaxivity and thence predict the pore-size distribution of the organic nanopores in kerogen, without additional experimental data.

  PublisherOA PDFDOI

• Dualistic Role of Alcohol in Micelle Formation and Structure from iSAFT Based Density Functional Theory and COSMOplex

  Industrial & Engineering Chemistry Research · 2023-01-23 · 8 citations

  articleSenior authorCorresponding

  Alcohols are effective and commonly used additives in surfactant self-assembling systems. Their effect on the behavior of these systems is complex and depends on a balance of the interacting forces. It is known that alcohols can act as cosolvents by altering solvent properties, and can also act as cosurfactants by inserting themselves into the palisade layer of the micelle and coaggregate with surfactants. The most recent models require user input to select the fraction of alcohols partitioning to each role. Two molecular theories iSAFT and COSMOplex are able to capture both roles and successfully predict the effect on micellar structure and critical micelle concentration of different alcohols ranging from short-chain to long-chain in qualitative agreement with experimental data. In this paper, the effect of adding linear 1-alcohols to aqueous solutions of nonionic poly(ethylene oxide) alkyl ether (CxEy) surfactant is investigated. To achieve this, the interfacial Statistical Associating Fluid Theory (iSAFT), a classical density functional theory, is applied to these systems and compared to the newly developed COSMOplex model and to experimental data. iSAFT and COSMOplex can provide detailed density profiles of each component in the system to illustrate the locus of an alcohol within a micellar structure. We observe from DFT that all alcohols studied are more present in the palisade layer than the micelle core, while COSMOplex shows greater accumulation of alcohol in the center of the micelle. Partition coefficients, aggregation numbers, micelle size, and total number of surfactant and alcohol molecules are also predicted and discussed. Furthermore, results from different surfactant architectures are compared and analyzed.

  PublisherDOI

Recent grants

• Microstructure and Properties of Inhomogeneous Polyatomic Mixtures from Density Functional Theory

  NSF · $300k · 2008–2012

Frequent coauthors

• George J. Hirasaki

  Rice University

  71 shared

• Francisco M. Vargas

  Rice University

  52 shared

• D. Asthagiri

  45 shared

• Doris L. González

  34 shared

• Arjun Valiya Parambathu

  33 shared

• Jefferson L. Creek

  Colorado School of Mines

  31 shared

• Prasanna K. Jog

  31 shared

• Bennett D. Marshall

  26 shared

Labs

• Walter Chapman Research GroupPI

Education

• PhD, Chemical Engineerin

  Cornell University

  1988

• B.S. in Chemical Engineering

  Clemson University

  1983

Awards & honors

• Humboldt Prize
• Donald L. Katz Award
• Outstanding Young Alumni Award from Clemson University