The first time I met János, I discovered a talented and charming young man. At that time renowned Centre de Mécanique Ondulatoire Appliquée (CMOA) of Professor Raymond Daudel which attracted a large number of scientists from all the parts of the world. János had come in the luggages of Imre Czismadia, one of the godfathers of the Hungarian Quantum Chemistry who was a go-between East and West. I do not exactly remember the year; nonetheless, our first meeting happened at the end of the 1970s because Janos was still bachelor. His first visit has been scientifically fruitful as he published a nice paper in Journal of Chemical Physics in collaboration with people from CMOA [1]. We rapidly became friends and I was struck by his interest for the French culture.

Later, János made several stays in France. In 1985–1986 he visited the Laboratoire de Chimie Physique directed by Professor Christiane Bonnelle in which Professor Raymond Daudel was hosted. Their collaboration on the study of molecules containing group 16 elements was materialized several articles [2,3,4]. During this stay I had the chance to work with him on the representation of solids [5]. I was amazed by the progresses he made in French; he was already able to understand and appreciate contrepèteries (l’art de décaler les sons), the French word for spoonerism. He was really gifted to speak foreign languages, maybe because Hungarians, as suggested by Henry Miller, have done the most difficult part learning their mother tongue. I helped him to discover French singers as Georges Brassens and Bobby Lapointe as well as Saint-Julien wine; in exchange he initiated me to Tokay and Hungarian musicians such as Bakfark Bálint. We travelled together to Torino to visit the group of Cesare Pisani and Roberto Dovesi; at the desk of the accommodation we realized that we were both born on the same day of the year. This fact is not so exceptional for quantum chemists since both Jerzy Cioslowski and Paul Popelier share the same birthday with us.

He invited me to spend a week in Budapest at the end of the 1986 winter, and I enjoyed the city and the Hungarian cuisine. I returned to Hungary in August 1987 for the WATOC, held in Budapest. I travelled with my family; János had reserved for us a nice house in Szigliget, a village on the north bank of Balaton where we spent 2 weeks of holidays before the congress. He found for us an apartment in Buda for the remaining of our stay. We met his family. János and Mercedes, his wife, did as much as possible to make our stay wonderful.

In 1989–1990 he got a position of invited researcher in my group in the Laboratoire de Spectrochimie Moléculaire. We worked together on the modelling of water molecules in solid state [6], and he published an article on the role of induction in infrared matrix shifts with my Phd student Hannachi [7]. Meanwhile, we used to play tennis, and he was much more better than I was, and he adapted his game to my poor level. Simultaneously, he initiated several fruitful collaborations with scientists of the Laboratoire de Dynamique des Interactions Moléculaires (DIM) of the University Pierre et Marie Curie, mostly with François Colonna [8,9,10,11]. This stay has been an opportunity for him to apply for a permanent position at CNRS. He left France to Germany where he got an Humboldt fellowship in Stuttgart and Bonn. He came back to Paris in order to satisfy the CNRS recruitment procedure, in which he succeeded. That is how he obtained a position in Jean-Louis Rivail’s laboratory in Nancy.

In 1993 I moved to the DIM and I have been involved in the creation of the “European GdR” CNRS G12090 “Dynamique moléculaire quantique appliquée à la catalyse, l’adsorption et l’absorption” initiated by Hervé Toulhoat where I was happy to regularly meet János. When in 1996 he defended his habilitation, he asked me to be one of the referees. The European GDR granted his Phd student Yannick Jeanvoine who defended his thesis at the end of October 1998. János asked me to participate as referee to the thesis jury. I came to Nancy with my wife and we stayed a few days at his home. His family had significantly grown since our travel to Budapest, and we were amazed by the size of kitchen utensils as well as by Mercedes’ skill to gently manage their nine children.

In 2000 we organized together the ChemBond meeting in La Colle sur loup. He found a very nice place to host the conference as well as the accommodation and the meals of the participants. The special issue of Theoretical Chemistry Accounts provides a full account of the level of the conference [12]. I remember that Janos who was a practicing Catholic posted the timetable of the religious services in the hall of the congress. At that time he looked still healthy, but he told me that he probably suffered of a fatal disease. During the last decade, his health problems increased, and 3 years ago he was hospitalized in Paris for a severe liver failure. I went to see him and I was desperate when I left him. Six months later, he was again working, involved in a collaboration with colleagues of LCT. He attended the meeting organized to celebrate Andreas Savin’s retirement, and I found that he had significantly re-established himself. Unfortunately, this only was a respite.

With János, I have lost a precious friend. The hours I spent in his company are engraved in my memory as hours of happiness. I know that he does not need any recommendation letter to obtain a permanent position in the Heaven of theoreticians.