UC Berkeley chemistry teacher Phillip Geissler, who took an entertaining approach, was known by many students as “the nicest professor they ever met at Cal,” a former student said.

Chem 1A is the introductory chemistry course at UC Berkeley designed to identify the freshmen who have the talent to continue on a premed pathway in a semester-long pressure cooker. But whenever Professor Phillip Geissler taught it, students could get at least a moment of relief when he brought in his guitar.

Standing before 500 students as if the Pimentel auditorium were a concert hall, he’d sing catchy chem-humor tunes, with titles like “The Mole Song,” and “Acids and Bases.” He’d even bring in a professional singer as a guest performer. It was the kind of extra effort that won Geissler the top campus awards in teaching.

“Students would often say that he was the nicest professor they ever met at Cal,” said Lucie Liu, a former graduate student in chemistry who was selected for the statistical mechanics dream team known as the Geissler Group.

In mid-July, he made a detour to Moab, Utah, for a day or two of hiking on his way back from Colorado, where he gave a lecture at the Telluride Research Center. Geissler was aware of heat danger and very careful, said his partner, Lauren Nakashima, so he set early on the morning of July 17. By 8 a.m., when he was at the visitor center, it was already 80 degrees, headed to triple digits.

When he did not check in Nakashima at home in Oakland, a search was mounted. His body was found near a trailhead two days later. The cause of death is pending an autopsy, but it was suspected to be a heart attack or other sudden medical event, Nakashima said. He was 48.

“He was overdue for a big American road trip to the wide-open landscapes to see that southwest geologic splendor that he loved,” said Rebecca Overacre, who described herself as life partner of Geissler, though he lives with Nakashima. It worked for them.

“The three of us believe in the infinite capacity of the human heart,” Overacre said during a joint call with Nakashima. “Phill was good at working through challenges, scientifically and otherwise.”

Geissler was a science prodigy who earned his doctorate in chemistry from UC Berkeley in just four years, when it normally takes five. The College of Chemistry at UC Berkeley is consistently rated in the top three nationally, and is among the best in the world. There is international competition for every tenure track position, and Geissler was the kind of talent who sparked a bidding war. Three years after leaving Berkeley to do postdoctoral work at Harvard and the Massachusetts Institute of Technology, he was back for good and on his way to a prestigious endowed chair as Aldo De Benedictis Distinguished Professor of Chemistry.

Chemistry Professor Phillip Geissler on the UC Berkeley campus in April 2022.

Starting in 2003, he moved quickly up the ranks from assistant professor to associate professor. While still in his 30s, he won the Berkeley Distinguished Teaching Award in 2011. One year later he was promoted to full professor. He became an internationally recognized expert in the field of statistical mechanics, which employs computations to study chemical systems.

“Phill was a truly remarkable person — a brilliant researcher and scholar, a phenomenal teacher and mentor, and a dear friend to so many,” Douglas S. Clark, dean of the UC Berkeley College of Chemistry, said in a statement announcing Geissler’s death. “His students found him to be both accessible and caring.”

Phillip Lewis Geissler was born March 27, 1974, in Ithaca, N.Y., where his father, Fredrick Dietzmann Geissler, was earning a doctorate in music from Cornell University. The family moved around, following teaching opportunities before settling in Richmond, Va., where Fredrick Geissler transitioned to computer science.

According to his older brother, Fritz Geissler, Phill excelled from his youngest school years in both math and humor. He picked up the guitar in middle school at the same time as his brother, who is four years older.

“Phill learned the basic chords, and once he had those, he could listen to a song and be able to play it all the way through, by ear,” said Fritz, an elementary school principal in Chesterfield, Va.

At Douglas S. Freeman High School in Richmond, Geissler played center fullback on the soccer team but quit to focus on his position as lead guitar in a rock band named Solomon’s Marbles after a jamming Grateful Dead instrumental. He grew his hair to shoulder length, wore tie dye and hit any Dead or Phish show within a six-hour drive.

He also developed an interest in hiking and backpacking. One summer when he was in high school, he and his dad flew to Utah to explore the high desert canyons, which he would return to right up until the day he died.

“He loved the colors and the sandstone formations,” his brother said.

As a high school senior, he earned a full academic scholarship from the Ethyl Corp. in Richmond. He chose Cornell for its top-ranked chemistry department and for its setting in the gorge region of upstate New York. He was also aware that a Grateful Dead show in the campus gym on May 8, 1977, is generally considered one of the band’s best all-time concerts.

After graduating summa cum laude from Cornell in 1996, he came to Berkeley. During grad school he got a call from Overacre, an old friend from Richmond who graduated from Virginia Tech and was thinking of also moving to Berkeley. She did, and they were later married in the Blue Ridge Mountains of Virginia.

They bought a house in Rockridge in 2004. That year, Overacre entered a relationship with Alison McLennan, an Oakland furniture maker, and she eventually moved in. Geissler later entered a relationship with Nakashima and moved out but not on. “We expanded into two happy households” is how Overacre described the relationship with Nakashima, who is the academic personnel analyst in the Office of the Dean at the College of Chemistry.

Geissler’s reputation as an educator attracted talent from around the world to the UC Berkeley program. Lucie Liu came from the University of Cambridge in England.

“I wanted to study with the best statistical mechanics professor,” she said, “and Phill Geissler’s group was my top choice.”

The Geissler Group is a mix of a dozen doctoral candidates, postdocs and even an undergrad or two who work together on projects.

“We are not party people. We are all nerds,” said Nathan Odendahl, who met and married Liu while in the Geissler Group.

“There is a legacy of people that Phill taught that are now professors in their own right with their own lab groups,” Odendahl said. “He was an extremely patient teacher and would explain a difficult concept in a way to make sure the student completely understood it, and not every professor is that way. Most are not.”

Most are also not the type to provide in-class musical accompaniment. During Big Game week, when UC Berkeley’s rivalry with Stanford is usually at its apex, he would have liquid in a beaker change from red to blue and gold.

At the penultimate moment, the doors of Pimentel Hall would fly open and in would march the California Straw Hat Band. It is a Chem 1A tradition going back 50 years, but no other professor would pick up the guitar and play along with the band the way Geissler would.

Geissler was serious about the instrument. He studied classical and Spanish guitar, and he was good enough to back Austrian singer-songwriter Clara Blume on a Bay Area tour in 2018 and 2019.

The tour ended on the UC Berkeley campus, in Chem 1A, a surprise performance.

“The students were absolutely baffled and overwhelmed by how thoughtful Phill was as a teacher,” said Blume, who has a day job as vice consul of Austria.

“Here is a guy who is universally acknowledged as a generational mind in his field, and he’s conveying complex content in the most approachable and entertaining way.”

Survivors include Geissler’s partner, Nakashima, and life partner, Overacre, both of Oakland; mother, Beverly Geissler of Richmond; brother, Fritz Geissler, also of Richmond; and three nephews.

Donations in his name may be made to the First Nations Development Institute, 2432 Main St., Second Floor, Longmont, CO 80501, or the National Parks Foundations, 1500 K St. N.W., Suite 700,Washington, DC 20005.

Sam Whiting is a San Francisco Chronicle staff writer. Email: swhiting@sfchronicle.com Twiter: @SamWhitingSF

Sam Whiting has been a staff writer at The San Francisco Chronicle since 1988. He started as a feature writer in the People section, which was anchored by Herb Caen's column, and has written about people ever since. He is a general assignment reporter with a focus on writing feature-length obituaries. He lives in San Francisco and walks three miles a day on the steep city streets.