Obituary of Thiru Thirunamachandran

Dr. Thirunamachandran, always professionally and affectionately known as Thiru, was born in the north of Ceylon (now Sri Lanka) into a Tamil family of six children. As an undergraduate in Colombo, his academic record was such that he was appointed to the staff even before his graduation results were known. In 1958 he went to University College London to pursue a Ph.D. in theoretical chemistry, under the supervision of David Craig. He returned to Colombo, after his Ph.D., in 1961. In 1963 he married and returned to UCL, and was appointed to the staff in 1965.

His Ph.D. work was on exciton theory of molecular crystals. He was the first to secure the lattice sums necessary for higher‑order multipole interactions. This was in the era of ‘computer centres’ where large rooms, banks of machines, air‑conditioning and rotating drums for memory were the norm. Thiru’s thesis was widely borrowed, and it was quoted in the primary literature no less than 13 times before its contents were published. His collaboration with Craig continued over three decades. It was productive and satisfying to both partners.

Thiru played a leading part in the development and application of non‑covariant quantum electrodynamics, an adaptation of Dirac’s 1928 general theory suitable for the interaction of radiation with particles moving at non‑relativistic speeds. The starting‑point in this development was a paper by Edwin Power and Sigurd Zienau, published in 1959 in the Philosophical Transactions of the Royal Society, on the Coulomb gauge in quantum electrodynamics.

With Power and others, Thiru showed that with the coupling of the radiation field to molecules described in dipole approximation the theory was readily applicable to absorption and emission processes, and to molecule‑molecule interaction as in resonance energy transfer and in dispersion coupling in both classical and Casimir‑Polder limits. His work also addressed chiral discrimination and induced optical activity, as well as higher‑order processes such as two‑photon absorption, which have become so important with laser sources.

Thiru wrote jointly with Craig a highly successful book entitled “Molecular Quantum Electrodynamics”. First published in 1984, it was reprinted by Dover Publications in 1998. The level was chosen to be suitable for graduate students, beginning with the elements of quantization of the free electromagnetic field, coherent and chaotic states, and quantization of the coupled system of particles and field. The particles were taken to be slow‑moving, as in molecular systems, and the problems discussed included two‑photon absorption and emission, scattering, optical activity, and interactions between molecules. There was a brief concluding section on self‑interactions such as the Lamb shift.

In a later paper with Power he showed how to recover the electric and magnetic fields of moving multipoles, by solving Maxwell’s equations without using the electromagnetic potentials or specifying the gauge. The first step was to get the Feynman expressions for the fields of a charge in motion. A key feature is that the method depends on the integration of a differential equation that is first‑order in time. Thiru’s work continues to be well cited both for his methods and results.

Thiru was always courteous, generous, modest, and self‑effacing, and when he retired he refused any form of public recognition of the event. When refereeing a paper, he would always work through the mathematical derivation from beginning to end, often coming up with a simpler solution. Typically, on one occasion when he showed that a derivation was quite wrong, he rewrote the paper from beginning to end. The author offered to make him a co‑author, but Thiru refused to be identified, and was thanked only as an anonymous referee.

The breadth and depth of his knowledge was remarkable. He was an excellent conversationalist, and any anecdote which you offered would be capped by a better one, whatever the subject. He had a special interest in the history of chemistry, particularly that of the UCL chemistry department, and in the lives of scientists. He was a family man, devoted to his wife Siva and proud, in an understated way, of the remarkable success of his sons Rama and Gopala in the academic and financial worlds.

Thiru retired in 1998, but he continued working daily in the chemistry department at UCL until he was taken ill at the beginning of August 2008. He died on October 13 2008, and leaves the memory among his many friends of a much admired and loved scientist and gentleman.