Pekka Pyykkö talks to Caroline Moore about relativistic effects and the thrill of theoretical chemistry

Pekka Pyykkö

Pekka Pyykkö is professor of chemistry at the University of Helsinki where he studies relativistic effects in heavy elements and simple chemical species using theoretical methods. He is an editorial board member of Physical Chemistry Chemical Physics.

Why did you become a scientist? 
I think I became attracted to science on my own. I kept a diary when I was a child and around the age of eight, I started making observations on nature and reading popular science books. I was interested in all sorts of science - chemistry, things like electric motors, electronics, and rockets. The last entry in my diary was at the age of thirteen: 'finally one of the electric motors worked and one of the rockets flew.'

What made you specialise in theoretical chemistry?
I did my PhD degree in physics but it was a very chemical kind of physics - I participated in building an NMR machine. My PhD thesis was about two thirds experimental NMR on solids and one third quantum chemistry, much of which I learned in Uppsala, Sweden. There was no official quantum chemistry in Finland, before I founded it myself. I sometimes joke that I went from being a Finnish speaking experimental physicist to being a Swedish speaking theoretical chemist!

You've done a lot of work on new gold species and on relativistic effects. Why did you found these so interesting?
While I was working on the theory of NMR properties, I realised that there were very significant relativistic effects influencing the heavy elements' spin-spin coupling constants, which no one had told me about. I started to try to calculate them and had a few early papers on the relativistic theory of NMR parameters. After we had started to do relativistic molecular calculations, I was comparing certain properties for silver and gold compounds and I realised that the entire difference between them comes from relativity, which was a revelation.

In 1979, Jean Paul Desclaux and I published, in Accounts of Chemical Research, the article that may have helped to sell relativity to chemists, together with a companion article by Kenneth Pitzer. I also published a relativistic review article in Chemical Reviews in 1988, which has been quoted over a thousand times now. 

I realised from the beginning that this was going to be something very important. The fact it explains something so simply that it can go to a textbook is what I would consider my largest achievement.

Computing power has increased enormously over the past few years. What difference has that made to the work you have done?

You can do far more complex calculations. One very good philosophy is that if you push your calculations to the currently available limit, more often than not, you find new science.  Another thing that made relativistic quantum chemistry available to anyone was pseudopotentials. There have been other approximate methods, but the market share of pseudopotentials is perhaps the largest one.

Do you think theoretical chemistry has a lot to teach experimental chemists?
Yes, I think so. One joke which I often quote comes from the famous Russian crystallographer, Alexander Kitaigorodsky who said: 'You have three levels of theory: third rate, second rate and first rate. First rate theories predict, second rate theories forbid and third rate theories explain.' It can be valuable to explain something like the difference between silver and gold but the best kind of theory is to predict. We have probably predicted a few hundred compounds, of which about forty have now been made. We have had a fairly good rate of success which I find most interesting. I think the theoretician is really leading the way. Half of chemistry is still undiscovered. We don't know what it looks like and that's the challenge.

Tell me about the winter schools on theoretical chemistry you are involved with in Finland.
In the darkness and cold of mid-December in Finland we have started to have these schools. The conditions keep the tourists away! We focus on some narrow subjects, for instance astrochemistry or actinide chemistry. It's done with very light administration: we want to know who is coming but we neither give nor take money. One Monday morning in mid-December suddenly we have a hundred scientists from all over the world spontaneously assembled. Quite often young people are coming to their first conference to show their first poster, including our own youngsters or those of our European partners.

How important do you think international collaboration is between chemists?

Crucial. I would talk about complementary competencies. From the beginning, I was always the idea man but I was not necessarily the best person to do the actual work. Getting the work done, especially if it is technically difficult, always means finding the best expert to work with.

Another area you're interested in is the history of science. How did you become involved in that?
I grew up in Turku, which is the former centre of Finland and the site of our first university in 1640. Chemistry was studied there from 1761. For many years, just per chance, I lived at the exact address where Johan Gadolin, the famous Finnish chemist, lived and, since 1984, I have been in the younger branch of his Chair at Helsinki. I find it simply fascinating.

Do you think it's important that younger chemists should be made more aware of the history of science? It's often a subject that's taught as part of history, rather than part of science.
I think the history of science has a very invigorating effect on students. If you want to have success in science, my advice is simple: aim right, hit hard. In order to aim right, you have to develop an intellectual taste. To do this, history of science is crucial because it shows you what ideas turned out to be important in the long-run. Another thing which is also fun is to see that these scientists were also human, as human as you and me.

If you weren't a scientist, what would you be?
Perhaps I could have had success in languages because I speak a number of them. Or I could have been a plumber. In our house, we recently had a very demanding plumbing problem. Its immediate importance for people who are freezing in the Finnish winter is far higher. Maybe I could have a second career!