Vibrational averaging techniques to calculate the role of water dimers in atmospheric absorption Jonathan Tennyson, Matt J. Barber, Ross E. A. Kelly, Lorenzo Lodi Department of Physics and Astronomy, University College London CAVIAR meeting Cambridge, Feb 2011 CAVIAR - Continuum Absorption at Visible and Infrared wavelengths and its Atmospheric Relevance

(1) Need to solve the nuclear motion Hamiltonian –12D problem! Approximations required. (2) Fully dimensional potential energy surface required –Huang, Braams and Bowman (HBB) potentials 30-40,000 configurations sampled. Calculated at coupled-cluster, single and double and perturbative treatment of triple excitations method. Augmented, correlation consistent, polarized triple zeta basis set. Polynomial fit with 5227 coefficients. Water Dimer Method HBB – X. Huang et al. J. Chem. Phys. 128, (2008). HBB2 – X. Huang et al. J. Chem. Phys. 130, (2009).

(2) Fully Dimensional Water Dimer Potential Monomer corrected* HBB potential Corrects for monomer excitation –Accurate modes for the monomer * S. V. Shirin et al., J. Chem. Phys. 128, (2008). R.E.A. Kelly, J. Tennyson, G C. Groenenboom, A. Van der Avoird, JQRST, 111, 1043 (2010).

Solving the 6D intermolecular problem Brocks et al. Hamiltonian*Brocks et al. Hamiltonian* Solved using Dimer code of Groenboom and van der Avoird: Gives dimer VRT states * G. Brocks et al. Mol. Phys. 50, 1025 (1983).

Solving the 6D intermolecular problem Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) In Plane Bend (IPB) Stretch Out-of-Plane Bend (OPB) Generated by Matt Hodges and Anthony Stone. C. Millot et al. J. Phys. Chem. A 1998,102,

Adiabatic Separation Approximate separation between monomer and dimer modes –Separate intermolecular and intramolecular modes. m D – water donor vibrational wavefunction m A – water acceptor vibrational wavefunction d – dimer VRT wavefunction

Now we can vibrationally average the potentialNow we can vibrationally average the potential Input for 6D calculationsInput for 6D calculations donoracceptor State mState n How well does it perform for |0 0> calculationsHow well does it perform for |0 0> calculations Solving the 6D intermolecular problem

In cm -1 Red – ab initio potential Black – experimental GS – ground state DT – donor torsion AW – acceptor wag AT – acceptor twist DT 2 – donor torsion overtone R.E.A. Kelly, J. Tennyson, G C. Groenenboom, A. Van der Avoird, JQRST, 111, 1043 (2010). Vibrational Averaging

Vibrational Averaging: 6D Costs! Computation: –typical number of DVR points with different Morse Parameters: –{9,9,24} gives 1,080 points for monomer –1,080 2 = 1,166,400 points for both monomers –1,166,400 x 2,894,301 intermolecular points = 3,374,862,926,400 points Same monomer wavefunctions for all grid points Distributed computing: Condor 1000 computers, 10 days But we have a way to probe high frequency dimer spectra

Full model for high frequency absorption Approximate separation between monomer and dimer modes Franck-Condon approximation for vibrational fine structure Rotational band model

Adiabatic Separation Approximate separation between monomer and dimer modes –Separate intermolecular and intramolecular modes. m D – water donor vibrational wavefunction m A – water acceptor vibrational wavefunction d – dimer VRT wavefunction

Model for high frequency absorption Approximate separation between monomer and dimer modes Franck-Condon approximation for vibrational fine structure Rotational band model

Franck-Condon Approx for overtone spectra Assume monomer m 1 excited, m 2 frozen m 2 i = m 2 f I (2) Franck-Condon factor (square of overlap integral): Gives dimer vibrational fine structure (1) Monomer vibrational band Intensity

Allowed Transitions in our Model 1. Donor 2. Acceptor All transitions from ground monomer vibrational states Assume excitation localised on one monomer

Franck-Condon factors –Overlap between dimer states on adiabatic potential energy surfaces for water monomer initial and final states –Need the dimer states (based on this model).

Transitions: Example Donor – Vibrational ground state (VGS) Acceptor – VGS Donor –VGS Acceptor – bend Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) Ground State (GS) Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) Ground State (GS)

Transitions: Example Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) Ground State (GS) Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) Ground State (GS) Donor – Vibrational ground state (VGS) Acceptor – VGS Donor –VGS Acceptor – bend

Transitions: Example Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) Ground State (GS) Acceptor Twist (AT) Acceptor Wag (AW) Donor Torsion (DT) Ground State (GS) Donor – Vibrational ground state (VGS) Acceptor – VGS Donor –VGS Acceptor – bend

Outline of full problem Need to ultimately solve (6D problem) H=K+V eff V eff sampled on a 6D grid Calculate states for donor Calculate states for acceptor Vibrationally average potential for each state- state combination –Really only |0j> and |i0>

(a) 6D averaging: (b) 3D+3D averaging: 1 C Leforestier et al, J Chem Phys, 117, 8710 (2002) 2 R. E. A. Kelly et al. To submit shortly. Averaging Techniques

Form of the wavefunction: –(I) Uncoupled free monomer –(II) Uncoupled perturbed (fixed) monomer R. E. A. Kelly et al. To submit shortly.

Problems with Fixed Wavefunction approach (uncoupled methods) Donor bend (Donor) Free OH stretch (Donor) Bound OH stretch (Donor) Free OH stretch (Donor) Bound OH stretch

Averaging Techniques Form of the wavefunction: –(I) Uncoupled free monomer –(II) Uncoupled perturbed (fixed) monomer –(III) Coupled Adiabatic R. E. A. Kelly et al. To submit shortly.

Averaging Techniques Form of the wavefunction: –(I) Uncoupled free monomer –(II) Uncoupled perturbed (fixed) monomer –(III) Coupled Adiabatic R. E. A. Kelly et al. To submit shortly.

Averaging Techniques Form of the wavefunction: –(I) Uncoupled free monomer –(II) Uncoupled perturbed (fixed) monomer –(III) Coupled Adiabatic R. E. A. Kelly et al. To submit shortly.

Averaging Techniques Form of the wavefunction: –(I) Uncoupled free monomer –(II) Uncoupled perturbed monomer –(III) Coupled Adiabatic Coupled Adiabatic methods are the most suitable –Requires wavefunction calculations at each intermolecular grid point! 2,893,401 * 2 DVR3D calculations! –So we use cheaper (3+3)D averaging technique. –Still costs! CPUs for 3-4 weeks. This part is complete.

Calculating dimer spectra with FC approach Solved for monomers Coupled adiabatic appoach Vibrationally averaged potential for donor- acceptor state-state combinations |0j> and |i0> Input for 6D intermolecular problem Now we can solve 6D intermolecular problem Obtain vibrational fine structure For T=296 K requires all vibrational states

G16 Symmetry of Hamiltonian for GS monomers –> replaced with G4 Greatly increases computational requirements So far Reduced angular basis Small radial basis 320 diagonalizations for 0-10,000 cm -1 Each at 16 GB 8 states per symmetry block Gives 20,480 transitions: results presented by Matt Solving the 6D intermolecular problem: Allowed permutations for excited monomers

Model for high frequency absorption Approximate separation between monomer and dimer modes Franck-Condon approximation for vibrational fine structure Rotational band model

G4 Symmetry of Hamiltonian Require all vibrational bound states Greatly increases computational requirements So far Reduced angular basis Small radial basis 320 diagonalizations Each at matrix 320,000 x 320,000 means ~ 1 TB RAM ~400 states per symmetry block 3 – 5 days CPU Ongoing (problems with UCL computers) Solving the 6D intermolecular problem: For atmospheric temperatures

New Model to probe near IR and visible regions of the water dimer spectra. –vibrational fine structure Aim for spectra for up to 15,000 cm -1 produced. And all states up to dissociation to be calculated. -- Computer resources a big issue Conclusions