Long Hoang Nguyen Theoretical & Computational Chemistry

How molecules interact from small clusters to the bulk

I study molecular interactions using ab initio methods.

PhD student in the Xantheas Group, Department of Chemistry, University of Washington.

Explore Research H2O Quantum chemistry Contact

Live simulation — hydrogen bonds forming and breaking

Affiliations & Facilities

  • University of Washington
  • NERSC

About

Intermolecular interactions in the gas & condensed phase

Everything I work on is a version of one question: how do molecules interact? The balance of electrostatics, polarization, dispersion, and charge transfer governs how matter behaves everywhere, from dilute gases to liquids and solids. Get that balance right — at quantum chemical accuracy, at large scale — and whole classes of problems in chemistry and materials open up.

Water is my flagship system, and deliberately so: its hydrogen bond network is the sternest test an interaction theory can face — cooperative, delicate, and unforgiving of shortcuts. My work in correlated and many-body electronic structure theory, using MP2 and CCSD(T), asks how correlation energy decomposes across such a network and how that decomposition can be exploited to reach system sizes canonical methods cannot touch. Bridging the gap between ab initio accuracy and large scale systems is where I work.

The immediate goal is a unified, quantitative theory of hydrogen bonded systems. The longer arc extends the same tools to large scale, chemically complex environments — ion–water systems and electrolytes, where dissolved ions reorganize the network around them; proteins and their hydration shells, where binding and folding are decided by the same intermolecular forces; and metals, oxides, and interfaces.

I am advised by Dr. Sotiris S. Xantheas (University of Washington and Pacific Northwest National Laboratory).

Advisor & Group

Dr. Sotiris S. XantheasXantheas Group — University of Washington · Pacific Northwest National Laboratory

Location

Seattle, WashingtonDepartment of Chemistry, University of Washington

Research

Key Research Thrusts

01

Scalable correlated binding energies for water clusters

Published · HF+MP2(1B2B) In progress · HF+CCSD(T)(1B2B)

Schematic of the HF+MP2(1B2B) and HF+CCSD(T)(1B2B) approximations A full water cluster Hartree–Fock calculation is combined with monomer and dimer correlation contributions to estimate an MP2 or CCSD(T) binding energy. Full cluster Hartree–Fock energy + Monomer and dimer correlation 1-body2-body EstimateMP2 orCCSD(T)
Concept: combine a full cluster Hartree–Fock calculation with one- and two-body correlation contributions, at the MP2 or CCSD(T) level.

Canonical correlated calculations become rapidly more expensive as a cluster grows. Splitting a cluster's binding energy into one-, two-, and higher-body terms shows that the Hartree–Fock part needs the whole cluster, while the correlation part is captured almost entirely by single molecules and pairs. Treating correlation through monomers and pairs alone opens the way to correlated binding energies for much larger clusters.

Technical HF+MP2(1B2B) keeps the full cluster Hartree–Fock energy, which retains cooperative three-body effects, and truncates the correlation contribution at the two-body term. In the published study, HF+MP2(1B2B) and localized LMP2-1M2M recover more than 99% of the canonical MP2 binding energies for the clusters examined. HF+CCSD(T)(1B2B) applies the same approach to the CCSD and perturbative triples (T) correlation contributions. These are calculated results for the reported test sets, not a universal bound for all hydrogen bonded systems.

Related publication →

02

Vibrational spectroscopy of solvated water

Published · J. Chem. Phys.

Solvation and vibrational-frequency schematic A water monomer is surrounded by progressively larger hydrogen bonded environments, while schematic bending and stretching bands shift with solvation. Increasing solvation environment MonomerFirst shellLarger network StretchBend Frequency shift
Calculated trend: local hydrogen bonding and progressively larger solvation shells reshape the bending and stretching frequencies of an embedded water molecule.

A single water molecule and bulk liquid water absorb infrared light very differently. This work follows how local hydrogen bonding and progressively larger solvation shells reshape the molecular bending and stretching modes.

Technical The published study evaluates model networks from the monomer through third solvation shell structures containing up to 53 water molecules, using MP2 and CCSD(T) potential energy surfaces with discrete variable representation treatments.

Related publication →

03

Transferable water potentials

Ongoing objective

Environment-dependent water potential schematic The same central water molecule is shown in cluster, interfacial, and bulk environments, connected to an adaptable one-body potential. Molecular environment ClusterInterfaceBulk Environment-dependent one-body term
Research objective: adapt the intramolecular potential to the local hydrogen bond environment while retaining a consistent many-body framework.

A water molecule's intramolecular structure and spectroscopy depend on its surroundings. A transferable model must represent that response across clusters, interfaces, and condensed environments without treating the molecule as rigid and unchanging.

Technical Ongoing work explores environment-dependent one-body potential and dipole moment surfaces informed by correlated calculations and comparisons with established many-body water potentials. Transferability across environments is a target, not a completed result.

Spectroscopic foundation →

Publications

Scholarly outputs

Peer-reviewed publications and the M.S. thesis. The published record is also available on Google Scholar.

Peer-reviewed Publications

  1. 01

    L. H. Nguyen, S. S. Xantheas

    Many-Body Expansion of the MP2 Correlation Contribution to the Binding Energies of Water Clusters: A Localized Excitation Analysis

    Journal of Chemical Theory and Computation 2026 · 22 (19) · 9888–9900 Published DOI: 10.1021/acs.jctc.6c01287

  2. 02

    L. H. Nguyen, G. D. Santis, S. S. Xantheas

    Environmental effects in the vibrational shifts of water: from the monomer to models of the bulk

    The Journal of Chemical Physics 2026 · 165 (7) · 074307 Published DOI: 10.1063/5.0340361

  3. 03

    L. H. Nguyen, T. H. Dunning Jr., S. S. Xantheas

    A Cautionary Tale: Failure of the Valence CASSCF to Describe the Hallmark of Hydrogen Bonding

    Journal of Chemical Theory and Computation 2026 · 22 (13) · 6283–6289 Published DOI: 10.1021/acs.jctc.6c00606

Thesis

  1. 04

    L. H. Nguyen

    Electronic Structure and Vibrational Spectroscopy in Hydrogen Bonded Clusters: Application to Water

    M.S. thesis · University of Washington 2026 ProQuest Dissertations & Theses Global View thesis

Presentations

Talks & posters

  • June 2026

    Electronic Structure, Vibrational Spectroscopy, and Many-Body Effects in Hydrogen-Bonded Clusters: Application to Water

    Flash talk & poster — Los Alamos Computational Condensed Matter Summer School (LACCMSS), Los Alamos National Laboratory

  • June 2023

    Quantification of Arsenic in Marine Organisms

    Poster — Erickson Undergraduate Research Conference, Seattle Pacific University

Education

Academic background

  • 2026 — present

    Ph.D., Chemistry

    University of Washington · Xantheas Group

    Theoretical & computational chemistry.

  • 2026

    M.S., Applied Chemical Science & Technology

    University of Washington

  • 2023

    B.S., Chemistry & B.A., Physics

    Seattle Pacific University

Awards

Honors & recognition

Selected academic, research, and training honors across the University of Washington, Los Alamos National Laboratory, Seattle Pacific University, and Wellspring Education.

  • 2026 — 2027

    Excellence in Chemistry Graduate Award for Incoming Ph.D. Student

    University of Washington

    Competitive award granted upon acceptance to the PhD program in Chemistry.

  • 2025 — 2026

    Lloyd E. and Florence M. West Prize for Best Thesis

    Department of Chemistry, University of Washington

    Awarded once per year across the M.S. program in Applied Chemical Science & Technology. Announcement

  • June 2026

    Travel Scholarship — Computational Condensed Matter Summer School

    Los Alamos National Laboratory

    Highly competitive research program in condensed matter physics, held over two weeks. Announcement

  • 2024 — 2025

    Honorable Mention — Lloyd E. and Florence M. West Prize for Excellence in Research

    Department of Chemistry, University of Washington

  • 2023

    Wes Lingren Award for Excellence in Analytical Chemistry

    Seattle Pacific University

  • 2023

    Excellence in Physical Chemistry Award

    Seattle Pacific University

  • 2022 — 2023

    Wellspring Undergraduate Scholarship

    Wellspring Education

Teaching

Teaching & Pedagogy

My teaching centers on conceptual diagnosis, active problem solving, multiple representations, accessibility, and a deliberate transition toward student independence.

I identify the conceptual gap beneath a student's difficulty, rebuild the idea through active problem solving and multiple representations, then reduce support until the reasoning is independently transferable.

01

Diagnose first

Separate conceptual, mathematical, representational, and procedural obstacles before choosing an instructional approach.

02

Connect representations

Link equations, molecular pictures, graphs, physical meaning, and computational results without reducing rigor.

03

Build independence

Use active practice, accessible scaffolding, and mistakes as evidence, then gradually withdraw assistance.

Direction: undergraduate mentoring, physical and quantum chemistry, computational learning materials, and bilingual Vietnamese–English educational content.

Skills & Software

Methods & tooling

Electronic structure

  • PySCF
  • NWChem
  • Gaussian 16
  • ORCA
  • Q-Chem
  • GAMESS
  • MOLPRO
  • OpenMOLCAS

Methods

  • Hartree–Fock
  • DFT
  • MP2
  • CCSD(T)
  • MCSCF / CASSCF
  • CASPT2
  • GW / BSE

Condensed matter & MD

  • Quantum Espresso
  • BerkeleyGW
  • FEFF10
  • DCA++
  • LAMMPS
  • FitSNAP / SNAP

Programming & ML

  • Python
  • Fortran
  • MATLAB
  • R
  • Bash
  • TensorFlow / Keras

High performance computing

  • Linux
  • Slurm
  • UW Hyak
  • NERSC Perlmutter
  • LANL Chicoma

Visualization & documentation

  • VMD
  • Avogadro
  • Jmol
  • GaussView
  • ChemCraft
  • MacMolPlt
  • LaTeX

Contact

Get in touch

LinkedIn linkedin.com/in/Jacques114 GitHub github.com/jackn1104 Google ScholarPublication profile ORCID0009-0006-8506-3095
Location Seattle, WA